This 3v3 SSG can be done without the 4 GK's, you can instead use 4 mini goals. 2 games are ongoing at once and as soon as a goal is scored in either game, the resting team take the place of the team who scored.  

The resting team must be aware of what’s happening on both pitches as they are required to join immediately.  This is a fun and fast paced small sided game.