In the vast majority of cases, a relationship that is broken with two participants is going to be broken with more than two participants. People don't suddenly become less obnoxious, more mature, less possessive, just because there's someone else around. If anything, existing problems tend to get rather amplified with the addition of more people.

The main case where this isn't true as much is when a relationship is otherwise good, but there are specific things that are missing -- there's not conflict, just things missing. For example, if one person in a couple enjoys BDSM and the other doesn't, adding someone that can fulfill that desire can be quite helpful.

There's always exceptions, of course, but that seems to be the way things generally work out.