I'm kinda with SomeGeezer on this one. It's a shame things didn't work out for the three of you but it would be cool to maintain a friendship. Poly people are not exactly a "dime a dozen" so just having someone as a friend that you could talk openly with about Poly topics would be great imho. Not only that but as time goes by and you really get to know each other a lot of the baggage of meeting someone new gets left behind and a person can start to relax and feel comfortable.
If I read your post correctly it is the new lady that is ending the relationship so the choice of whether or not to maintain a friendship may not be yours to make. But I thought I would add my two cents as a general comment about Poly situations.