Don't ask questions you can't accept the answer of.
Don't let them go on about their exes too much. It sucks to be broken up with or lose someone you really cared about by betrayal. But you and your fiance set yourselves up for failure by making this the status quo for three whole years. Make some boundaries on the issue. He needs to process this shit and move on before the wedding.
Lessons I've learned the very hot way:
Don't ask questions you can't accept the answer of.
Don't let them go on about their exes too much. It sucks to be broken up with or lose someone you really cared about by betrayal. But you and your fiance set yourselves up for failure by making this the status quo for three whole years. Make some boundaries on the issue. He needs to process this shit and move on before the wedding.