The threesome handbook has really good advice about navigating jealousy and other people. While it's technically geared for threesomes it definitely crosses over very well.
The Ethical Slut is the Bible though.
This is a deeper issue than can be solved with a Reddit post. It stems from personal insecurities, but is a normal reaction for some people. You might not be ready for swinging.
There is a book called The Threesome Handbook by Vicki Vantoch. It has an excellent chapter on navigating insecurities and jealousy in non monogamous situations. I highly recommend you start there.
There is an entire book written on this subject that I highly recommend.
It will teach you to navigate jealousy, introduce new ideas, and much more.
I still read it and I'm a pro now. :)
I've never done it before but here's a great book about it how to not only manage the physical aspect but the emotional aspects of it. It's definitely something that you shouldn't do on a whim and definitely needs to be fully discussed. What are your limits? His limits? Hers? What's okay what's not? Having her join you and your partner after a breakup WHILE living with you could be a recipe for disaster. So don't do this lightly.
https://www.amazon.com/Threesome-Handbook-Practical-Guide-SLEEPING/dp/1568583338
I've no experience with threesomes personally (other than discussions), and everyone else has given some sound advice. I just want to say this:
Baby steps.
You can baby step it.
If you met someone tomorrow that you were both into, that doesn't mean you have to doff your clothes and jump immediately into bed. You can cuddle pile, kiss, try oral and stop there (or even before). There's no rule that says penetration has to happen the first time out or even at all (a lot of people, especially guys, get hung up here). The only objective rule is consent. Everything else, you make up as you go along.
Also: https://www.amazon.com/Threesome-Handbook-Practical-Guide-SLEEPING/dp/1568583338
Haven't had a threesome, but the general consensus for them to be successful seems to be: communication, communication, communication, transparency.
That is, you and your partner go into the threesome knowing why each person wants it (e.g. they're not just trying to fuck someone they have a crush on without it being considered cheating); knowing what each other wants out of it (sleeping with the same sex, watching partner, etc.); knowing what your boundaries are (e.g. no anal with anyone, maybe you take the lead and your boyfriend joins in at your request, etc.); knowing that if anyone (including your third) calls a halt to things, everything stops; being willing to communicate and engage in aftercare as a couple (and to also communicate with your third partner and ensure they're okay), etc.
>am open to the idea though not thrilled about it.
I don't think you have to be thrilled but you do have to be interested and invested. If a threesome holds no real interest for you, don't just do it for your partner. That way often lies disaster. Keep thinking about it, figure out if it's something that appeals to you, for whatever reason.
And after that, if you decide to proceed, go very slow (baby steps) and have boundaries in place. By that I mean....let's say you find a partner that you both want to take to bed. Then you make it a boundary that you only go so far on your first evening together. Maybe that's a cuddle pile with some kissing while watching a movie. Then you see how everyone feels after that and decide if you want to go further. There's no need to jump right into the deep end.
ETA: You might want to check out this book, as it discusses dealing with things like communication and jealousy: http://www.amazon.com/The-Threesome-Handbook-Practical-SLEEPING/dp/1568583338
Generally speaking (I'm not someone who's had a threesome - I'm just speaking from research, common sense and anecdotes), I think if anyone is prone to jealousy, threesomes are not a good idea. That said, if someone really wants one, then - at the very least - it will take some groundwork to figure out when/why the person gets jealous and then determine - and enact - a healthy way to approach that jealousy.
There's a book by Vikki Vantoch, called The Threesome Handbook (http://www.amazon.com/The-Threesome-Handbook-Practical-SLEEPING/dp/1568583338) and she has a chapter on communication (e.g. what each person is hoping to get from the threesome) and dealing with jealousy and potential negative fallout. It might be worth a read for you two. (Also, don't forget to consider the possibility of your jealousy. There have been plenty of stories of guys gung ho to watch their partner with another woman....only to find out that, hey, they got jealous....because their partner was fucking someone else.)
Another area where I think people fuck up (leading to the disastrous threesome stories) is they identify someone to have a threesome with and - thinking this will be their only ever chance (which, okay, realistically, it might be) - they dive right into the deep end. There are no baby steps. No everyone kissing or cuddling and then waiting a little while to let that settle, it's off with the clothes and straight to bed. That might work for someone people; for a lot of others, it seems to not. So, long story short: in addition to lots of communication, taking it slow is probably not a bad idea. That gives everyone time to adjust to the new dynamic and discuss any (unexpected) feelings that come up.
Get the threesome handbook
https://www.amazon.com/Threesome-Handbook-Practical-Guide-SLEEPING/dp/1568583338
I highly recommend reading this book. It covers so much more than you can get here.
Start with this book first. And read it together.
You both should work on your trust issues on overcoming infidelity turmoil. AND if you ever wanna venture into threesomes for real...
Here's TWO books ( this one should be read no matter what) - <em>Not 'Just Friends'</em> by Shirley glass Not only useful for after an affair but a great guide on how to build a stronger relationship between you and your partner so that you can prevent infidelity and increase true intimacy.
If you wanna learn more ways to have a healthy threesome experience. - The Threesome Handbook: A Practical Guide to SLEEPING WITH THREE
" The perks of three: hotter sex, more love - Strategies to prevent freak-outs, jealousy, and general messiness - How to find the perfect third - Discovering your inner queer - When a ménage turns into love—how to create a successful triad relationship - Communication skills for couples - Knowing when to break it off - And much, much more. Drawing on personal experience, historical research, advice from pros, and hundreds of interviews with veteran and novice threesome adventurers, Vantoch covers the gamut of possibilities in threesome sex and triad relationships. The Threesome Handbook is the essential road map for couples and singles to explore new territory or just spice things up in the sack."
This book has some good talking points about communication and how to deal with potential issues (e.g. jealousy): http://www.amazon.com/The-Threesome-Handbook-Practical-SLEEPING/dp/1568583338
I havent read it
This book is awesome.
https://www.amazon.com/Threesome-Handbook-Practical-Guide-SLEEPING/dp/1568583338
Best book out there on threesomes, esp. for married couples