i just put a 3s 2200mAH flightmax lipo in my DX2.0, works fine.
here's my post:
http://www.rc-monster.com/forum/showthread.php?t=19233
edit: i forgot that the DX3r uses only 4 double a's, but i have seen people convert it to use 6 or 8 IIRC, im sure you could use a 3s, but i would check with the manufacturer to be sure that it won't mess anything up.