Pulling off a 1v3 is basically impossible; it depends far more on the trio’s mistakes than the solo player’s skill. I think giving each player one revive would be a good change, along with making surviving players stronger whenever an ally dies.
A quick map pool voting system after the teams see each other.
Balance queues based on loadout value, too; if you spend 3k on a build, you shouldn’t get matched against three players in full green gear.
Make it easier to reuse builds; I don’t think players should have to play almost every class or use a lot of green gear just to repeat a build—using around four different ones should be enough.