A bit ago, I unlocked the third party slot. I quickly filled it with a not-really-lower level pet, thinking that the dungeon would be easier and I would be more powerful with that third pet. My other two pets were fairly "high" leveled at the time, high twenties or mid-thirties, and the third one was in the mid to high twenties. Before I'd filled that spot, my duo was pretty powerful-ish [but were only getting one or two exp from battles].

Surprisingly, it took much longer to finish a battle against two or three enemies than it had been with just the two in my team. It seemed like their usually powerful attacks had had their power knocked down. I took the third pet out after a couple battles that went like that.

Was it just me? Or what? Does filling extra party slots alter things like amount of exp given or the strength behind attacks like Bash and Bolt?

The dungeon is an incredibly difficult zone. It is meant to be more challenging and feel 'end game' to any particular zone so it is just that the difficulty of the area you were in is amped up a TON in comparison to the other places in the zone.

@Yumi123

There is a slight decrease to the amount of Experience earned if you add another pet to your Party. However, it has no affect on the difficulty, amount of enemies, or anything else related to the actual battle.

Hope that helps.