I wish I could put a vote in for "I've done it and enjoyed it" AND "I've done it and haven't enjoyed it" - for the most part, I have found it enjoyable, but in my experience, it has always been problematic in some sort of way on my end, or the couples' end.
I have been in relationships where my partner wanted to have a FFM or MMF, and I just did not enjoy any of them. It was 100% consensual on all ends, but I was left feeling insecure just seeing someone else with my partner or knowing my partner got off watching me with someone else. I can write an entire list of why, but that sums up those scenarios. I don't care to ever have a threesome again as someone who is in a relationship. There is a lot I am "into" into the bedroom, and when I am involved in a relationship, I do not care to share my partner in any way sexually. It just really does not turn me on, but I know many couples that truly LOVE the thought of their partner with someone else or being turned on by someone else.
-BUT!- When I was single, I was involved as a third for many threesomes, and I thought it was SO much fun. There -IS- a "right" way to do it. I literally had rules set out. It made me feel like I was getting action, but not getting emotionally attached in any way to anyone or doing any work to find someone. I felt it to be so freeing and I felt sexually empowered. I loved it. You can't get involved in any way and you have to just accept yourself as an "accessory." When you get in deeper than that, you will end up disappointed. If you can get your mindset right as a "3rd" it can be SO much fun.
^^With all of this said, in my experience, it always ended up with one of the parties involved in the "relationship" part of the threesome messaging me or trying to contact me outside of our "dates"??? It always made me uncomfortable, and I would make sure to tell the other person (NO secrets, ever!) and that pretty much ended everything and it would cause a huge fight between the couples. One of my rules was that if any of that were to happen, I will NOT take sides and you won't hear from me again. That's what happened. I always made it clear that I never wanted to be the reason to potentially split a marriage or a relationship in any way.
This is my experience. I could write pages and pages of my personal experience on threesomes. Overall, I've found it to be problematic for the couples involved, not necessarily the "3rd." I feel I've had a fair amount of experience on both sides to say it's been problematic. (In MY experience.) Unless there was a way for the "third" to have no connection/name/means of communication outside of the bedroom, I think that's the only true way to enjoy it and keep it 100% SEXUAL.
So, for the sake of not writing a BOOK, my vote is just "No...unless you can be the third when you're thriving, single and not giving AF." lol
I'd love to see other's experiences! Thanks for posting, @Emily!