Thanks for the replies.
Okay, let me add that I know that for some it comes naturally. I feel for me it might, although it hasn't come up yet. But how about those who it doesn't come naturally for? Say there's a SO who agrees to open the relationship, but not partake in the openness. They need to "find" compersion. And although we'd love to believe that it comes as easily as "I want you to be happy" would, it probably doesn't.
So, was it just as easy for your partner(s) to feel compersion? For the non-poly side of a poly relationship, does it just "come naturally," or did you have to work on it? And if so, was there anything in particular that helped you/them?