The only real answer is it depends on the group just like it would depend on the couple in a monogamous relationship how they handle breaking up.

In our situation I have kids with both men so there couldn't be a situation where in we simply tell the one to go away. We have a responsibility to the children, no matter what age they are, to behave in a mature manner and continue to co-parent.

If we were to break up we would have to negotiate the split just as carefully as any monogamous couple has to during a divorce. It would suck, be painful, and change us fundamentally...just like breaking up does for any configuration.

Polyamory isn't really anymore complicated that monogamy there are simply more people involved. Break ups are messy and painful no matter how many people are going their separate ways.

You all need to come together as a loving unit and set some new goals and negotiate what has changed, what needs to change and where to go from here. I would suggest sitting down in a neutral setting like a coffee shop or restaurant...or even a public park and talking about the future. Relationships become strained when communication breaks down. Pull together as a unit and I predict the rest of the stuff can be worked out.