Unlikely to find a cheap 3A rated boost board.
Normally LiFePO4 2S packs are used for RC receivers and servos, these are 3.2V per cell so 2S is 6.4V and find with servos.
I'd normally budget 1A per small servo in the absense of a datasheet, so 6V 4A seems safe - any RC LiFePO4 2S pack should be able to source 4A. 3A may be plenty though, it all depends on the actual servos, which alas often have no useful datasheet.