Can you use Charm to move an enemy unit to a battlefield occupied by another enemy in a free for all game?

Yes, you can use Charm to move an enemy unit to a battlefield occupied by another enemy unit in a free-for-all game.

According to RiftJudge FAQ #7385 and Rule 444.2, movement caused by card effects like Charm is not restricted by the same rules as a Standard Move (such as the Ganking requirement). As long as the destination is a legal battlefield (i.e., it does not already have units from two other players present, per Rule 444.2), you may move the unit there.

In a free-for-all game, moving an enemy unit to a battlefield occupied by another enemy unit will cause a showdown between those units, and the opponent who controls the unit you moved will become the attacker in that showdown (as clarified in RiftJudge FAQ #7385).