My boyfriend wants to try cukolding?

Just tell him NO. This is HIS male fantasy. HE wants a threesome. HE thinks this will bind you to him and you already know that it won't. You are already having second thoughts about him. He whinges about the size of his penis and dumps his insecurities onto you so you can worry about them and keep stroking his ego to make him feel better about himself. HE will be the one afterwards (when you are feeling rather used and dirty) to either tell you it was great and when will you do it again with a different guy - or oh dear, YOU shouldn't have done that because it's ruined our relationship. I think you should think VERY carefully about whether to stay with this boyfriend of yours. I wonder if this concern about YOU having had only one partner means that it's HIMSELF he's worried about. Has he had any other partners but you? After 3 years are you in a rut perhaps?

These are tremendously divisive fantasies. Your BF might say he is ready and wants to try this, but once the time to do it actually arrives, he may not be so sure anymore. Also various jealousies might crop up after having done this. He may now think that you prefer men with bigger penises than him. There would be nothing you can say to dissuade him from this belief, because it's just that, a belief.

Avoid it like the plague, and even get mad at him if talks about it again.

Tell him to forget it. If he can't, forget him. Given your reservations, that is not something you should consider or do. If you go against your principles, you'll never get over your regret.

I'm not very experienced with this and I could be wrong but I've had this brought up to me quite a few times and my assumption has been that the guy who brings this up wants to hit the sheets with another man.. but needs a girl there so it's not "gay". Still seems gay to me.

Look, if YOU want to experience another man then break up with your boyfriend and go try out someone else. But for you to accept joining in on threesomes with two guys because your bf wants to when he's the only person you've ever been intimate with? Sounds like him taking advantage of you and trying to be your ruler.

He wants to find out if his penis is as big as the next guy's? He can go to any public bathroom and find that out. You don't need to be there.

Threesomes sound good and are great to fantasize about but in reality they can cause a lot of problems for a couple. The biggest issues are doubts about themselves and the relationship. Common ones are:
'Am I good enough for my partner? Am I bad lover? Will they give my partner more pleasure than I can give them?'
'Will he/she still want to be with me or are they now wanting to be with that other person?'
'I'd rather be with that other person. What am I going to do now?'

To deal with this, lots of couples have certain rules about what the person joining in can and can't do. And open communication is super important. Couples have to be honest with what they feel and whether they want it to continue.

Do you ever masturbate for him with a dildo? If you're okay with doing this, it might end his desire for a threesome. If it doesn't and he still wants to try this, you need to be honest with him and tell him how you feel about it.

He's an insecure twit. You should LEAVE him as soon as possible & be with someone who isn't so weak & willing to be exploited. Only ill can come to you from such a "relationship."

no clue what that means