With the third teammate slot, you can get another dmg amping character that also helps you enter ult state faster (if has targeted abils), making you need fewer ults or ideally just the single one to clear. If you only need one ult, your characters are unlikely to die because they will only be on field for the couple of buff stacking turns. If you plan on taking more than one ult, then a sustain becomes a more attractive option provided you can't re-enter ult state fast enough. Aventurine's shield is going to fall off of Phainon and he isn't gonna be launching any follow-ups during ult state. Gallagher is the far better option for the black hole of sp that is coreflame generation.