We spent half the 3v3 night chasing footballs

Yes — spare balls everywhere. I’d put several around each pitch and make it somebody’s job to keep them topped up. If one disappears, use the next one and carry on rather than sending an adult on a 30-yard retrieval mission while six children wait. With such small pitches the ball will go out a lot, especially while everyone is learning the format. Quick access to another ball makes a massive difference to how much football they actually play. You can collect the strays between games. This is one of those things that sounds ridiculously simple, but good 3v3 organisation is mostly about removing little delays. Pitches ready, balls ready, teams ready — then let them play.