Simple single leg Toe Touch to start. My PT, during ACL rehab, had me pick up 30-40 marbles at a time. I found that huge and I would do that daily. That evolved into Single Leg Deadlifts, but i still do the high rep/volume marble pickups for the whole hip down stability. The other two i like to use a thera-band just above the knees and do side-walks and air squats. Make sure when doing the sidewalks your in an athletic stance, like a goalie in soccer.

Those are my 3 big ones for developing good strong glute-med’s for my clientele, not to mention when worked properly, helps with mechanics too.