Telling them now is a good idea, not only for the reason you mention but also because the older they are when they find out, the more betrayed they may feel, as in, "I have a half-brother? And you didn't tell me? What other secrets are you keeping? How can I trust you any more?"
Tell them in a matter-of-fact way -- without getting dramatic -- and keep it simple. By simple, I mean that they only need basic facts: "There's someone in our family you have never met. His name is X, and he's 19 years old, and he lives in Y. He's your half-brother. That means daddy is his father, but I'm not his mom." I know that feels like half the story. I is. At their ages, that's all they need to understand. It's also about all they can absorb.
After you say that, allow for a pause. Then tell them, "If you have any questions, just ask."
The 3-year-old will likely not have any, or if he does, it's likely to be unformed and ego-centric: "Are you still my daddy?" Answer that even though it doesn't get asked. "Of course, I'm still your daddy [good time for a bear hug] and I love you very much and I always will." The 6- year-old may have questions, but they will likely be very concrete and may even seem weird: Does he go to school? Does he like ___? (Fill in the blank with his favorite food.) Answer the questions truthfully if you can. If you don't know the answer, say so. Remember that kids are ego-centric. A question like, "Where's he gonna sleep?" is really, "Is my bedroom still gonna be my bedroom?" Answer it this way: "You'll always have your bedroom. He's not coming here to live."
If you act upset or weirded out, they will be, too. If you present this in a "There's-all-different- ways- to- be- a- family" kind of way, they will accept it that way, too.
The point of this first conversation is really to get the facts on the table. As they reach new stages of development, there will be new questions because they will come to grasp the facts and implications in different ways. If a question comes up out of the blue, that's a good clue that he is reaching a new understanding. Some day, they'll want to know about this boy's mom, and whether dad was married, and if he's coming to live with them, and where will he sleep, and can they see a picture.