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.
Toys:
Slash with VXL
E-revo with MMM/Teckno Nue 1515/1yf running 4s and 5s