Relationships often change.

You might use them as exs.  Or as folks who your Main Character was previously involved with but they're now taking a break.  Or moved to being just friends for a bit.  Or they had to move for work.  Our MC had to move for work.

Or maybe their poly relationship was broken up when one of those characters fell for another character.  Whether that relationship is still ongoing or if it ended and they just didn't resume their poly relationship.

They could be good friend characters for potential 1x1 partners.  Perhaps they still have a thing for your MC but they're no longer romantically involved.  Perhaps they love or hate your 1x1 character.  They could either be allies to them, letting them know issues your MC has and supporting them if there are like hard times in the RP...or maybe they're a foil and they tell your 1x1 partner secrets about your MC or otherwise try to stir up trouble.

You might also use them to just show back up after a 1x1 has been going on for awhile, if it starts to slow down or you need a new sort of plot to develop.  Right when everything is finally going well in the 1x1 the old flame comes back from let's say London and shows up to the door to catch up on old times, only to run in to your partner.

There's lots of possibility.  Are they supportive, antagonistic, or just neutral.