When we moved out of our old place and into our new one, I thought the bunnies would get the hang of their box again since there weren't areas so saturated with urine that they automatically think to use them. That didn't work out very well. Then we built them a cage and they at least kept their messes to one area of the cage. Once they got their own room again, litter box training went right back out the window with all my hopes and dreams. Once our flemish giant joined the group, instead of them picking up his flawless litter habits, he picked up theirs. Recently we replaced the litter boxes completely with a HUGE one to accommodate Gobi's enormous size and the other two bunnies together. Nope, they barely touch it. We switched that litter box with Mrs. Brown's box in hopes that they'd try to mark it as their territory and start using that instead. Even worse.
I can't cage the trio again, Gobi is simply too big to fit in a cage let alone with two roommates.