The 3 glute muscles move the legs in different directions. To isolate each one requires a different movement.
Gluteus maximus - Lie on your belly and lift one leg while pressing the hips towards the ground
Gluteus medius - Lie on your side and lift your top leg to the ceiling
Gluteus minimus - It is really hard to isolate this muscle but if you;
Lie on your side
Lift your top leg and externally rotate the top leg to point your toe and knee at the ceiling,
Do tiny circles with the leg slightly behind your body you can find it.