Well all the pooping and spraying is normal for a new bun coming into the area. They want to make it theirs and, of course, your buns want to take it back.
This will die down as the buns become acclimated to one another – especially if they bond.
I am not experienced with bonding three rabbits but I have seen some bonding practices where they put all three together at once in neutral territory. But I believe that introducing them one at a time is the preferred method.
The mounting can continue for quite some time. In fact, I think some have gone on for a year or more. However, it is usually a quick thing just to remind them who is in charge. Unfortunately, sometimes nipping comes into play (or in my case, is always there). And when trying to establish a bond between two males, that is a chore as they will both want to be the boss. All you can do is continue to be regular and persistent about bonding as it can take months to attain.