Did you have a bad crash? A servo could be stripped although it's unlikely.

Changing the TX batteries is a good idea. I don't remember what the low battery cutoff is, but it's in the manual.
The TX beeping at you is a good indication that your batteries are indeed too low.

IMO, adjusting the gain pot on the 3 in 1 would not address a servo twitching problem but it would a tail twiching problem.