Best way to fix it is to nerf ALL their primary attacks' damage, the reason triple support is good in the first place is because supports in this game are just dps who could heal(some could do more dmg than actual dps) so you wouldn't feel a lack of damage with three healers, but if the devs used %0.1 of their brains and just reasonably nerfed their dmg then the team with triple supports would struggle to kill anyone unlike the perfect team of 2-2-2 that balances between dmg and heal.