I've stayed at both the Poly and the YC, and I've visited the WL several times.
First, I think they are all great resorts.
Second, do you know which parks you plan to spend most of your time at? Because YC is very convenient to both DHS and EP, Poly is close to MK and EP, WL is close to the MK... That could make a difference.
Third, it sounds like you will be there during the xmas holiday decorations. IMO that pushes the WL up some since the decor there really lends itself to xmas theming.
And as others have said, December is iffy for swimming, so I wouldn't base the decision too much on the pools. YC has the best pool (and several jacuzzis so even if it's not warm to swim you can still go out and relax). WL's pool doesn't thrill me, and the Poly doesn't have a jacuzzi but the pool is quite nice.
Also, there is a pretty big price difference between the 3 resorts -- that may help you to decide. The Poly rooms facing the castle are really nice, especially for watching the highly fireworks, but they are pricey.
Good luck with your decision! And don't worry -- they are all great places to stay!
ETA: Not sure if you know that the rooms are not the same size... WL will be the smallest, YC the middle, and the Poly rooms are quite large compared to those two. It may not matter much since there are only 3 of you (we are fine with any of them) but just in case thought I should add this info.