Ok thanks, this is helpful!
First, neutral space is very important (it might be the most important thing, in fact). The fact that they are getting a long in the current space (which I would call semi-neutral) is very encouraging, but I think you'll need to find a truly neutral area to work with them. Did you get them all at once? That would explain why it went from neutral to not-neutral.
If you aren't already, continue swapping all three among the different cages every day or two.
Mounting sometimes happens in even bonded buns. Do you see cuddling, sharing litter box without issue, sharing food without issue?
Does mounting ever lead to fights that need to be broken up?
For trios, you can work on each pair of buns individually (this is often preferred if one pair in particular is difficult), but since the dynamics in a trio are different, it can work best to work with all three at once.
The fact that they are cuddling in one of the cages makes me think maybe they are bonded? Or very close to it?
Until they are 100% bonded it is really important that all sessions together are closely supervised. Your approach so far is pretty unconventional, but it seems to have gotten you pretty far if they are cuddling?
Whenever I've gotten stuck in a rut with bonding, a change of location has helped, along with stressing. You can take all the buns for a ride in the car in a bin together, and set up a large pen at a friend's house, or in another area of your house. For a trio, two or three pens linked up would work great. Anything within site or smell of their home area is not neutral. They may just need some long sessions in a true neutral space to cement their bond.