The cage was entirely new and no piggy had been in it before you put the three of them in it on Sunday, if I remember it rightly. If so and therefore, the cage was the neutral territory bonding pen. That part is fine.
What you should not have done is put them all in the cage together, then put grids in and separated them, then removed the grids.
Once you’ve put them together then you should not separate them. Doing so interrupts their bonding. It takes two weeks of permanently being together for a relationship to fully form.
If you do separate them during bonding for any significant length of time, then you can’t just remove dividers as they will have begun to set up their own territory in their half of the cage. Thereby simply removing dividers and allowing them to wander back through the whole cage can be seen as a territory invasion. After a separation, any rebonding needs to be done somewhere other than the cage.
Hopefully you didn’t separate them long enough on Sunday for this to be the issue though.
You need to establish whether you are seeing normal dominance - nipping, submission squealing etc - which may seem rough to us, but to them it’s very normal and does not mean a bond is failing.
Or
Whether they are in fact not compatible and can’t remain together
