I tried the threesome thing with my ex girlfriend years ago and it worked great because her and I had been together for a while, the guy we got involved with was a very good mutual friend. Everyone knew that it was just pleasure and fun. However with my last male LTR we tried it with a couple (his best friend and his girl) and the guys ended up getting very jealous of us girls spending time together outside of the bedroom, because they figured when we said we were going to the mall we were really sneaking off to have sex, which was never the case. In the end that turned into a big huge mess, my ex hated her, her and her man ended up breaking up for reasons that had nothing to do with the situation, I got very close to her man (who was my ex's best friend all of his life) him and I ended up getting together, and they stopped being friends, we broke up. It was a huge disaster. (And I fell very much in love with the guy, but it was just too weird)
These are newlyweds but have they been together long? I mean, I wouldn't dare do it in a new relationship/marraige, but if you have been with someone for a long time and have exceptional trust and confidence in your SO and are just trying to explore new things, then it CAN work, just not recommended most of the time.