I do 3x3, but that doesn't has much to do with frontsquats, its just because i do them on the middle day of a 3 day split, so I go for less volume and to give my glutes and hams a rest (front squat is more quad specific) so they are ready for regular squats and deadlift on my 3rd day. I do regular squats 3x5.
However in the last 6 months I do feel like my front squats have improved considerably using this rep range, tho that is confounded by the fact that I do other lower body exercises, so hard to say if it was the brilliant rep scheme that is good for my frontsquat.