Vandersteen 2Ce Signature vs 3A

"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.

You could try, but that's gonna be a pretty comprimised setup. The Vandies are designed for a minimum seating distance of 8'. Any closer and the drivers don't blend the way that they're supposed to. With 12' of space, you're looking at placing the speakers 2' away from the front wall and you sitting 2' away from the back wall.

I don't know your exact room layout, but placing the vandies along the short wall may be an option. Perhaps placing them about 7' apart and sitting the minimum 8' away? You would probably need to toe in the speakers just a little bit, but Vandies don't seem to have as much trouble being placed close to side walls as some other speakers do.

Either way, in a room that size I would go with the 2CE Sigs, personally. Sure, the 3A's could work, but it just isn't enough space to do them justice, imo.

Bang for buck the vandies are the best if you like the sound it is very smooth, detailed and refined. Theil is also good but more $$$ for a similar sound. You just need to listen and see what you like the best. But to get the sound of the 3's you are looking around 10K+ new. I looked for other things because I didn't like the look wnated more wood. But the sound finally sold me.

The distance thing you are going to run into with all speakers not just vandersteen. Some smaller speakers might do a little better but won't sound as good as the vandies overall and vandersteens image so well that I think if you are tight in the space by just a little it is still going to be better than most of the stuff out there.

What amps are you running and cables? These also make a big difference in the imaging. If the amps are not up to the 3a level you may want to look into them instead of the speakers. I put a set of quicksilver v4 on my 2cesigs and wow more open and a better image.

I don't know how Vandersteen suggests measuring listening position from the speaker. Dunlavy and Thiel suggest measuring directly from listening position to the the center of the woofer (though their phiolosphies part ways in other regards). I think Green Mountain suggests measuring from the listening position to the the center of the distance between speakers "equi T". If Vandersteen's suggestion is similar Dunlavy and Thiel, your Vandersteens should fit in comfortably. If Vandersteen's suggestion is like the "equit T" things might get a bit tight. Having heard many of them, in many different situations, I personally find Vandersteens to be very sensitive to rooms. I don't know if there is any correlation, but, since Thiel followed Vandersteens lead in using passive radiators, they too now seem to be garnering this reputation. This was not as much of an issue previously with Thiels (though associated gear, especially amplification has always been a concern). All in all I think you'll be OK. I strongly suggest you give Vanersteen a call. Richard Vandersteen has always been a straight shooter.