There are so many individual things, Abby, that make a child fearful of the toilet or the process that you would like him to follow. Your experiences with your older youngsters was great, but apparently this 3yo is seeing the world and things like this differently and it becomes up to you to meet his needs and fears the best way you can. Getting away into a closet or into a corner is thus just a creative way for him to be alone where noone else can see him or interact with his desire for privacy...There are, in my opinion, three circumstances here which come to mind, given the information in your post. these are not to be taken as solutions for you, as you must yourself relax and take charge to help your boy. REMEMBER he is but 3yo and really a baby still and needs to be nurtured as such....You noted in your post that you tried punishing and rewarding(to no avail). This immediately suggests the lack of being consistent. Although punishment is out-of-order, you have to get a routine and be consistent whatever it is.He will remember being punished and no wonder to hide from you. He does see the things about him in his world differently than his siblings and you and I....Even more to the point, I suggest you continue toilet training. You will have to dig out his fears and gently work with him to overcome his apprehension towards the toilet, if that is the problem. Let him have his corner or closet and understand his need. Will not go into toilet training(too many little nuances), but work him gently back to the toilet. He should continue with his own little potty. He is still so young you know and your expectations should be downsized for this youngest one....And lastly, in keeping with the above, consider putting him back into diapers, especially for his travels into seclusion and until his training suggests otherwise. Along with all of this, I sense that your 3yo son does want to be a baby a little longer with all that comes with it. That is the world he knows and wants to continue in.....You will need to put these items together, schedule them to suit circumstances, love and (((HUG)))alot and keep calm to carry them out.