Oh, wow, sounds like you are all living on top of each other, dz! Yes, the biting will decrease as he gets older, partly because he'll be able to talk and partly because he will be able to control himself and his urges.
Re: toddler biting.
wow, go here, to the link below! Zero to Three is a national organization dedicated to infant and toddler care, education, etc. this is a great article, with links about reasons why, what to do to stop, etc. they have written what i started to write. http://main.zerotothree.org/site/PageServer?pagename=ter_key_social_biting
I would REALLY observe your son and see WHEN he bites, WHAT the SITUATION is, and then you can see what kind of biter he is and you wlil learn how to decrease the situations he is in that result in biting. For example, you mention that you cringe when he comes near, which means he's biting alot. Does that mean he's biting for attention, that's the only way he gets attention? I'd imagine that's possible, as he's one of twins and has a 3 yr old sister so there are lots of chlidren and two busy parents. OR, it could be as a function of his quick trigger emotionally. Perhaps you can redirect the response, but you won't stop his trigger from being quick. So if he's quick to be angry, you can say you can't bite, but you can stomp your foot and say No! then stomp your foot and say no to show him. He can learn to stomp and say 'no!' which would be a marked improvement to biting. In a year you can refine the stomp/no routine to something more socially appropriate!
But I'm also concerned, frankly, about your very physical 3 yr old, who is teaching her little sibs to be physical - after all, she headbuts and grabs them by the throat. Honestly, given your son bites and your DD1 physically hurts when upset, I'd only allow them to be together if an adult is supervising, seriously. this is called shadowing, and my teachers do this when they have a child who bites or hits or pinches, because only by being ON TOP of the biter/hitter/choker can you make sure the incidents decrease. Because every single time she hurts them, or he bites, it reinforces the hurting/biting as a solution.
I know you are disciplining both, that they aren't "getting away" with it, but in a way, they stlil are. Especially your DD1. Just her doing it over and over reinforces it again and again. So she grabs and chokes DD2, you put her on the couch (where you all hear her carrying on for 30 minutes and your lives are all impacted negatively with stress, etc. and she KNOWS this), and she gets lots of attention, doesn't she? In fact, the you talk to her after she does this thing, giving her more attention....
Instead, when she does her IMMEDIATE timeout for hurting the twins (and there would be NO WARNINGS for this type of thing), I'd have it be in a room where you are NOT. She can sit on the stairs, if you have any, or she can sit in the bathroom, or in the hallway. Not the living room, because how can you all enjoy playing when she's carrying on? And then when she's done screaming and rolling about, etc. I'd ask her (in a nice tone of voice, because you are modeling being nice and calm) "Are you ready to be gentle with your sister (or whomever she hurt)?" And until she can say "yes" calmly, she'd not be allowed to come back. Period. If she say "nunh! with that head shake of which 3 yr olds are so good at, I'd say 'sounds like you aren't ready to come back. stay on the stairs until you are ready to be Gentle Anna."
When she says she can be gentle, I'd say "Oh, I'm so happy you will be gentle! We love to play with Gentle Anna!" and, yes, I might think of a term to use like Gentle Anna (or whatever her real name is, lol) so she can identify as that person.
In fact, I'd go further. Because having her get out of control actually doesn't teach her much. What you want to do is to get her to recongize when she's getting out of control, and having her learn how to get back into control. So, for example, I'd stop her play as it gets a little frustrated and start naming this "you are getting upset. I want you to be Gentle Anna. Do you think you can? Or would you like to sit with me and relax for a few minutes so you can be Gentle Anna?" (say this like it's a treat, not a punishment to sit with you - it's all in the tone of your voice)
And then if she says she can play gently, let her keep going, but if she starts to ramp up again, I'd say "You know what? You aren't being Gentle Anna. Please come with me and let's relax so you can play gently." Then I'd take her by the hand and lead her somewhere you two can snuggle and talk quietly. This is NOT a punishment, it's a way for her to learn to take herself away to calm down, a way to learn to breathe deeply and calm down. then, when you can feel her body begin to relax, for her breathing to be deeper and calmer, you can say "Oh, I think Gentle Anna is back, hurrah! Let's go play with the farm house again." And then she can return to the play, no harm, no foul.
she MUST be taught to take deep breathes and calm down, and she'll only learn to do it if you can see the signs of trouble and start to teach her to stop and breathe and relax. Instead of taking her away, which might cause her to erupt into a tantrum, you could instead say. "Oh, I see that you are getting frustrated - your voice is getting loud and you are beginning to push. (or whatever you are seeing - NAME those things you see so she can recognize trouble, too). You need to get into control, let's take a deep breathe in, now out. Like this - and you two do deep breathing in and out for a few times together, she'll copy you. She should start to relax, then you can say "Oh, Gentle Anna is coming back, good job! Let's play again."
this will take LOTS OF WORK on your part, but honestly, you need to get your daughter into control, and help her learn to get into control. Her being so physically dangerous with the twins isn't going to get better, and she will always be bigger than they are, and the next time she really might hurt her sister or brother. And you can't let that happen. And believe me, she'll also play this way with other friends, and you don't want this. Period.
I'd also start doing the breathing with your son. I don't think timeouts are effective at this age, I really don't. You need to model what you want your son TO do, and you guide his behavior by being there, right by his side, stopping him when he gets really ramped up, etc. So when he's getting upset, you can say "you are getting upset, your voice is getting loud. take a deep breath, like this" and then you do it, facing him. he'll probably mimic you, he's just a little guy and will think it's fun. and by him doing that, he'll stop being so fixated on whatever he was fixated with.
But honestly, I'd have her play alone whlie you are making dinner and can't be supervising all 3 of them, etc. then I'd allow all 3 to be together only when either you or your husband are right THERE. and, yes, it's a pain in the neck, and your husband and you will have to take turns doing the shadowing 'i will play with the kids this morning while you run errands, then when you return you will play with the kids while I clean the house." But I'd say if you really turned your attention to extinguishing all physical behavior in your house you'd have made a MAJOR dent in it within 2 months.
So by April 15th. That's not too far from now. Go for it, really, you'll be thrilled you did all this work when by this summer your house is more peaceful. Because I really don't think your 3 yr old is going to just snap out of her physical responses to your twins, I really don't. And every time she does something like head butts or chokes, she helps your son ramp up and bite.... vicious circle!