This page has instructions on how to register for an open beta test for Rustbucket Rumble that will kick off on Steam on Monday. Here's a description of the game's three-on-three battles:

Rustbuckle Rumble is a 2D, arcade-y, team-based shooter. Inspiration for the mechanics was taken from classic run and gun games mixed with more modern, class-based, multiplayer shooters. It’s capture-the-flag where the players are the flags.

Games are fast as you battle your opponents, stun them, and then return them to your base, recycling them for parts before the other team can stop you. Your base uses the robot parts to create a giant robot super weapon that is unleashed to destroy the enemy’s base. Each game ends with an epic giant robot battle.