Which 3s pack?

[skellyo]

You'll want something ~28mm thick or less if you want to easily use the stock battery bar. 25mm is the max without using spacers under the rear bar hold down and unscrewing the front a couple of turns.

From a fitment perspective, I like the size of the PolyRC 3S 25C 4350mAh. http://rclipos.com/PolyRC25C.htm