Yes I agree with Lil too but have a little more to add.

except for a small percentage, 3somes are detrimental to a significant S/O relationship.

It opens the door for suspicion and jealousy. If you really love and value someone the best bet is to stay momogamous.

Your involvement would not enhance your ex's relationship and if you have design's on him then use a different tactic. This one could blow up in your face big time.