Most of us will have some experience with 3-somes. Personally, i have only limited experience (and one experience with a 4-some) and I can't say I liked it. It is just too complicated and the feeling of intimacy and concentration on the (sex)partner vanishes if there are more than 2. That's my personal opinion.
Now YY and ZZ have taken the concept of 3-somes a step further. I have written about YY and ZZ in the past and I recommend that you re-read this post to get an idea about their relationship. Now recently ZZ has met a new guy and he has "managed" the new guy (let's call him XX) into his relationship with YY. In fact, now most of the time XX is in YY's and ZZ's house and they have created a real "Ménage à trois" It is clear that XX is more interested in ZZ, and that ZZ is very much charmed by the attention of the (young and cute) XX, although YY told me that he and XX are also getting along well and are having sex. They have been very open about their relationship and all of the friends have followed events in this complicated relationship. YY keeps a brave face although I can just sense that he is jealous and feels like the one who loses the most in the new set-up of their household.
Yesterday in the bar, ZZ took me apart and asked "tell me honestly, what do you think about our new relationship?" I was very careful with my answer, as it is their life and who am I to judge them, but I told him that he needed to be very careful or he might hurt YY and lose his 14-year long relationship. I think ZZ understood what I was saying, and of course he must have thought about it too, but for the moment he is enjoying the excitement of a new guy in the house, and will continue with the relationship. If it works out well in the long term, good for them!