as noted by most the points above, if you do not have a tie breaker in place already than there is no way you can enforce one at this point. you let all three teams play and the most points takes all.
but before next season is would be VERY wise to create a tie breaker system. For my league, it starts with their Head-2-Head record. If that doesn't work than it goes to total points, then record against seeds #1, #2, #3. If by some crazy miracle there is still a tie, they get a coin flip.