We can't really comment, because we don't know how tight you two are as a couple. You could have the type of relationship that surmounts jealousy and can withstand anything. Or you could have a house of cards in a gentle breeze and one strong gust could bring it all crashing down.

You both need to be absolutely brutally honest about your insecurities, and discuss with each other the likelihood of disaster. What's the likely trigger point of a relationship stuggle? Are you scared she'll find your friend better than you?

Saying "not too long after me and the ex started having them we split" is a little like saying "I changed my breakfast cereal and a few weeks later my dog got run over". What makes you think the threesomes caused the breakup? I would argue the threesomes may have been the catalyst for an already impending breakup - their were cracks in the relationship to begin with, and the threesomes simply made them wider.

__________________
You know, Blackadder, for me socks are like sex. Tons of it about and I never seem to get any.

My Photos:
https://imgur.com/a/80hiRdm