By default, HH already has a 2.5x experience multiplier. It is completely expected that fights that were previously giving 175k-300k XP per fighter (with an XP booster active) are giving 350-600k in HM (also with an XP booster active).
Don't believe me? Calculate the XP you should be earning yourself!
- Add together the Fighter Score of all three opponents -- this is the base XP value of the node.
- Divide by 3 -- XP from 3v3 fights gets divided evenly across all three members of your team, even if someone's at Level 60 already.
- Multiply by 5 (Holodeck Mayhem XP multiplier)
- Multiply by 2 (if you have an XP booster active)
- Watch for the Longshot bonus on the results screen -- a bonus of +30%, +40%, or +50% means you should multiply your XP estimate by 1.3, 1.4, or 1.5 respectively.
The result should be within a few hundred XP of the amount the game actually gives you, due to the rounding in the Fighter Score display for fighters with over 10K score.