Vandersteen 2Ce Signature vs 3A

I owned 3A sigs for quite awhile. When I first received them I was working on a project in Dallas and set them up in the little condo the company rented for me. There was only enough room to set them about 2 foot from the front wall and the couch was probably about 6 feet from the center of each speaker. It wasn't even close to enough room for them.
Once I got home and put them in my normal listening room I found that they sound best when you can sit at least 8 feet from the speaker. Plus they need to be at least 4 feet from the front wall and, of course, you want space behind the listening position. I would think that for them to be at the best that the minimum length of your room should be 16 feet.

"I measured. The room is 12'X 17'. Is this big enough to accomodate the 3A sigs on the long wall"
I didn't use Vandersteen's method of speaker placement for your room. But, simply using the "rule of thirds", which is good for quick calulations, would lead me to beleive that your room is to small for the 3A's. That probably could be overcome by having some type of diffuser behind the listening position and the seating right up against it. But, I don't think that you would be getting the best from the speakers. If you use the Vandersteen method for setup, you need to find a speaker/seating position that is at least 8 feet from the speakers. That would put them very close to the front wall, which also could be overcome with some treatments along the front wall and may need bass traps in the corners.
If it were me, I'd maximize the room for the 2ce sigs that you already own rather than change for the 3A's. I've owned both plus the 5A'a and I think that in the proper setting the 2ce sigs are so close to the 3A's that improving the room and proper positioning will gain you more than switching speakers.