Oh poor Pika, he is really missing out! (Assuming he is a he? It's worth checking his sex here
I'm sure you've worked hard to make the cage interesting for them, but I would suggest you remove almost everything for a while - once all the platforms and other things are out, then Pika will not be able to separate himself off.
Leave just a toilet corner on the ground floor, and their loose substrate with a little nesting material. You should find that the three of them snuggle in a single heap to sleep, but if they don't, keep the cage bare until they do.
Then once they are consistently sleeping together, add a single hammock. They should then move into that together, and Pika will hopefully start taking his brothers'(?) lead.
Gradually add more furnishings, but I would start with things that can't be used to sleep separately, like ropes, perches, and sloping tubes.
I wouldn't bother returning any shelves or ramps to the cage at all, as they don't contribute to a challenging active environment, and usually just get peed on and smelly. Look in our set up section
for some ideas of layouts.