Channels are part of the IRC protocol specifically to solve this problem of "more than two people" communication. It's a direct part of the very essence of IRC, even though it's a very implicit concept.
Joining a "private" channel (+p, +s, or +i) and inviting two other people is probably the best way to have "three way private messaging" anyway, because no one would ever need to feature request "four way private messaging"-- you just invite the fourth user to the channel.. or the fifth, or the sixth, etc.
