My words literally betray me.
Hi, OI wanted to know how to play with 3 players. Do we have to play with 4 heroes whatever happens or is there a variant rule for 3 heroes?
There is no variant rule for 3 players.
My recommendation if you just want 3 characters in play.
Pick 1 character on the 1 player side. Pick 2 characters on the 2 character side.
Use the 4 player rules with these exceptions:
The group always splits in the same way, (the solo player will always face cards solo.)
Track experience separately for the solo and the duo.
There might be other adjustments I'm not thinking of, but I think that will do the trick.