You can't force them to get along, there's just no way. In fact, feeling like they're forced to be in a relationship that's not working for them may be a major source of the tension. Why can't you date them each separately? Triads seem much less workable a lotta the time than vee's.
This may be a matter of needing to sit down all three of you for a major heart to heart, everything on the table but with some ground rules for communication to keep it from deteriorating into another one of their fights... and with the understanding at the outset (don't want to spring anything on anyone) that things aren't working as is, and that it's time for a radical rethinking of how y'all relate to each other. What is strong and should be built up? What is not working and should be let go? Change is not the same as failure and is probably exactly what is needed.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.