I was thinking about using the 5965MG and 5985MG for 1/8 off road and was wondering if it'd be a good choice? I read that digital servos weren't good for off road though from some sites. Any comments?

Also will this work with the Spektrum DX3R just fine?

Either of those servos would be ok to use for steering or the 5955TG, but I would use a analong servo for your throttle/brake. i.e 965mg or the 985mg

I'm not aware of any incompatibility issue there.

why use analong for the brake/throttle?

will it affect my ability to control the throttle/braking curves with my Tx?

It wont have any effect on your curves. Analog is better for this application because it has a little "give" to them. Digital servos when used here tend to heat up and burn them selves out quicker if not set up properly. The analog servo will take more abuse.