I know I'm young and all ( 18 ), but I also know what it feels like to be in love and to feel hurt/confused/etc from it too.. I've been with my current boyfriend for a little over 4 years now and we've been through A LOT. He's 3 years older so it's been quite an experience with the age difference, but things in my life forced me to grow up way before I should have; so I've always been mature for my age. I really think this is the guy I'm going to marry. Here's what I think..
A relationship is built on trust.. You need to have a lot of it in both each other and in the relationship for it to survive. Especially when you're planning on devoting your lives to each other, you'd better trust the other person completely.. or what will keep you together? A lot of the things that some couples base their relationships on (like sex, money, looks, etc) don't last.. And when they realize it, their relationship is finished.
It's fine to have those suspicions sometimes because those feelings you have could really be true.. But I think you have to take into consideration how he might feel/react when you confront him if they aren't. I would be upset if my boyfriend accused me of something I didn't do, too.
And I think snooping is wrong.. No one wants to be snooped on for any reason. But to be honest, I might have that urge to if I felt my boyfriend was going behind my back.. I think the best thing to do right now is to come clean about it. Be honest and tell him exactly how you feel. He will probably be mad that you abused his trust, but then again, he's guilty of doing the same.
As for the threesome suggestion, that's just wrong on his part. My boyfriend used to ALWAYS bring up his ex (not in the way yours has) and I would hate it, but he'd never get the point when I would just stay quiet. He finally stopped talking about her after I told him straight up that it hurt me that he was still thinking about her, and even had the nerve to talk about her to me. But we definitely didn't get along..
My biggest advice to you is to just be completely honest to him about how you feel.. I think, as women, we just have to accept sometimes that men will be men.. They will occasionally look at other women, think about other women, maybe even flirt. But we can't get on their cases all the time unless we absolutely do not EVER do it ourselves. You never know how much a seemingly innocent smile to another cute guy could hurt your man. Anyway, so be honest! If he isn't the least bit considerate or sensitive to how you feel about what he does, he's just an ass.. No woman should have to live with someone who doesn't truly care about her.
My boyfriend and I have had our share of ups and downs, and what's made our relationship go SOO much smoother is being able to talk about everything. If I have a problem with something, I let him know. He does the same.. We do this with anything and everything, and I think it has saved our relationship.
I can only give you my 2 cents on this. I don't know you or him personally, and I'm definitely not a part of the relationship, so I can't tell you whether to stay or go. It's cliche, I know, but listen to your heart. Only you know what is best..
I hope this helps.. I know either way it's not going to be easy. Just putting myself in your position hurts me. I'm sorry you're going through this, Sweetie.. Best of luck to you.