This is the wrong sub to ask about this because the majority here are just going to try and convince you they always end horribly, since those are the stories that get posted here. Sure, there is some obvious risk involved, but there are plenty of success stories as well l. You're just more likely to hear about them in the more sex-positive subs.

To answer the question(s) though, yes my wife and I have had many threesomes (and foursomes!) up to as recently as a month ago. Has never caused any problems in our relationship. If anything, it has strengthened it. Still going strong for 16 years and counting.

As in just about everything in life, YMMV.