I voted for "Tell them exactly how I feel," but I'd also add that you should be really cautious in this situation because, like the others have said, this doesn't seem like a stable or mature couple at all. It's crazy that they will sleep around on each other just to get back at each other. What might they then do to you?
If you aren't currently being hurt by them, or at least not so much so that the good times aren't enough to make it worth it, then I guess you're justified in remaining onboard. However, if things start becoming excessively hurtful toward you personally, then you should start thinking about leaving (and should probably inform them of a time frame or something on that order).
I'm actually not sure how you're being affected by all this, or what your feelings are overall in the relationship. Ultimately, only you can decide whether it's worth it to you to stay. But things seem messed up enough to warrant being very honest with them about your feelings, at least.
Love means never having to say, "Put down that meat cleaver!"