Yesterday, I was at Busan fighting the thieves and the bandits, and my mercenaries are now at level 4! Here's what I've found out about Busan. Please let me know if I'm wrong.
The thieves and the bandits give 21 XP for each mercenary per battle. From my research, how much XP that they give does not depend on how many mercenaries fought in one group. I've sent all 6 of my mercenaries in one group and each mercenary got 21XP each. Then, I've sent 3 mercenaries in one group and 3 mercenaries in other group and they all got 21XP.
What this means is that the you are way better of sending all six mercenaries in one group rather than sending 2 groups of three. Why? Because your mercenaries will use less of SP (I don't really know what this is, but if this becomes zero, then your mercenary can't fight) and you will pay less for the hospital fees.
So, I highly recommend sending all of your mercenaries in one group!
