I kind of started this in the general Q&Asection, but maybe it should have gone here.
I currently have two wonderful bunnies. Evee, an approx 6yr dwarf hybrid, and Ash, also approx 6yr lop (maybe French). Both were rescued seporately (first Evee, then Ash about 6mths later). They bonded very quickly and are very happy together. I have been thinking lately more about getting another rabbit, a baby flemish giant, but have concerns abouthow it will go over with them. I didn't plan on bonding them as a three-some (I'm hoping the baby will be more human oriented), but do want them to get along and eventually share the same encloser (night/no ones home). Ash is really relaxed, but Evee can be more high strung at times (our little Diva). How should I approach this?
P.S. Both buns are fixed, as will any future additions as soon as they are old enough.

Just my own opinion, at their age, they are happy together and get along well, I would not add another to the mix and just enjoy them and let them enjoy each other. They are getting up in their senior years soon and I think another rabbit, baby or adult would just be too much for them to have to deal with especially if Evee is more high strung.

I agree with Sarita. Also, something your post caught my attention. You said you don't want a three-some, but you do want them to share the same enclosure. Bunnies can never share the same enclosure unless they are bonded and then they should not be separated. Bunnies are extremely territorial and won't let any other buns into their environment until you have worked to show them that this new bun is part of their "warren" (for lack of a better word). They're not like dogs and cats who can live together casually and play together but not be super bonded. They would either have to be bonded as a three-some or the flemmie will have to be kept completely separate and allowed separate play time.

I agree with Sarita that it's far easier to let your two live in peace and get a baby once they are gone.

Elrowhen's correct about sharing as well - rabbits just don't do this - it's all for one and one for all and most of the time, it's super stressful for owners and rabbits when it comes to bonding.

I would definitely not rock the boat. So much can go wrong when introducing a third bunny, including the break-up of the existing pair.

A few years ago, we played around with the idea of getting a third. A friend of ours brought the bunny over our house, and within moments, our pair were chasing and scuffling with each other, pulling fur and biting viciously. And this was just from the scent of a strange bunny in ANOTHER ROOM. We took the bun back out of the house and they were fine.

If you have the time and space to have the rabbits completely separate, it's something you can consider... but also keep in mind that a cage won't be adequate for a flemish giant, so you'd really need a separate room and exercise area. Never get a third on the condition that they HAVE TO bond, b/c you're setting yourself up for heartbreak.

They're not like dogs and cats who can live together casually and play together but not be super bonded

While I think this is true for the most part, this is exactly what my boys are doing. They are bonded to the point where they have out-time together and they get along. But Ive tried the cohabitation and it causes way too much stress on Stormy. He becomes incredibly aggressive and because hes a senior, I dont want to stress him out.

But I definitely agree with whats been said. There is a good chance your pair would never accept another bunny. I have 3 bunnies and my female, Olivia, is the type of bunny who could probably never bond with another. She becomes super aggressive around other rabbits. You never know what their personality will be when you bring them home, especially a baby.