I would suggest getting an imaging ultrasound to check for previously mentioned labrum tears, or any loose bodies in the GH joint, as well as shoulder capsule thickening or tear, and any other problems, since there are quite a number of possibilities of what could have happened ot all those three occasions. A more precise time scale would also be important. Also, is it painful? if yes,when? Lifting your arm to the side, over your head, reaching a back pocket,sleeping etc. as much info on how the problem presents itself would be great