Hi TC... 3Ds are totally elective.  THe hospitals usually have the technology to do them, but won't unless they are medically necessary.  I chose to go to a company called Goldenview Ultrasound that is located right in Coolidge Corner in Brookline.  There is another place in Watertown and a few others in MA.  We paid $150 ... so it is certainly a splurge.  For that, we got several color printouts, a dozen black and white printouts, and a DVD of the whole session (~20min movie).  I thought it was a supercool technology and knew it was a total "extra" but something that I really wanted to do. 

http://www.goldenviewultrasound.com/