I think you're right about "every guys dream", I know I used to dream of a 3-some with me & two girls...... never did get to try it either! But, with age comes (a LITTLE bit of) "real-life"!!
Last things first - definitely DON'T rush into it, especially in a LTR like you have..... I reckon it's a bit like getting a tattoo - quick & easy to do, but you have to live with it for the rest of your life!
As far as thinking you're "not enough" for him - I really don't think it's that at all... more likely it's a fantasy he has, and one which he wants to share WITH YOU!! If he knows you are interested, that may be why he's brought it up in the first place?!?!
As to whether he'd be satisfied with "just you"..... well, that's something that neither of you will know until/unless it actually happens! I had a MFM 3some a few years ago, with my ex-wifes best friend and her husband... it was totally spontaneous (I think!!!) and I enjoyed it a lot more than I would have imagined.. BUT, we ended up sleeping together by ourselves later, and they have since split up... don't know if that had anything to do with it, but...........
Jeez, ALL my posts today are turning into novels!! I think what it boils down to in the end is - Do you trust each other enough to try it, and walk away if you have ANY reservations? And, have you thought about who the third-party will be? THAT could be the most awkward aspect of the whole idea!!
Good luck with your decision..... and if you DO do it.. come back & tell us what happened!
DM