Awww geezzzz, I didn't mean put HIM in the shower! I meant put the litter box in the shower IF you didn't have enough room in the bathroom for it on the floor somewhere!
He needs to get fixed. Can you borrow some cash from family, credit card, something to get him done? It may not calm him down right away but it will help. Not to mention the damage that has been done could be just because of hormones.
Baby steps are what is needed. Put him in a room where he can't do damage. Strange that he was ok with you at night but now he is doing that much damage. You need to start over with the introductions if you want this to succeed.
You need to keep the kitties separated 24/7 for a little while. The two dominants are pulling a number on him that is just not fair or right. Attacking him in the litterbox is going to mean lots of issues in the future. He will get to the point where he will not use the box!
Did you look at the site I gave you? This woman has lived it! She knows what she is talking about. Quite a few of us have as well.
It is important that you start over with the intros and go s l o w l y!!
FYI - when you take him to get fixed he will smell differently than he did before he went to the other two. A little baby powder on your hands, rubbed together, and petted on all three of them will make them smell the same. Or use a little vanilla under the chin.