[2021] All 3 tanks killed, what now?

If you have no more spawn - you can leave the battle. You will get additional RP/SL after the battle ends (you can check this in Economy tab in game).

If you lose all your spawns before everyone else, maybe try to change your tactic? Do not rush so fast on enemy, stay with your team. Many times it's better to score few crits and assists than one quick kill on enemy.