Baby bun being introduced to bonded pair

Hi all! I have been reading through the bonding section for quite a while now and I have a basic idea of how I am going to approach this.
Right now I have a bonded male/female pair. They are very gentle, non-moody buns. Their bonding was quite instant, so I think we all really got lucky there.
Within the next couple of days I plan to attempt to bond a young bun (from a rescue I work with) into the group.
This new bun is about 3 months old, is female and not yet spayed. No worries-she will be spayed as soon as she gets a little bit bigger and the vet is comfortable with her having a GA.
My current pair are both spayed and neutered.
It's the initial introduction that I am fuzzy on. I have portable pens, and can reduce the size to whatever is recommended. Also, I will of course be using neutral territory. But again - what is recommend for the method of the initial introduction? Separate pens for the pair and the new bun so that they may sniff at one another? If so, for how long shall I do this? If not by that method, is it recommended that they initially be kept completely separate and instead mix some littertray smells between the pair and the single first so that they can get used to the other's scent? Do you suppose I will have better luck attempting to bond a pair with a youngster as opposed to an adult bun?
Oh, also my current bonded pair are free-hopping house buns. If I am to put them in a pen under any circumstances I'm sure they'll be right peeved!
Any advice for a total bun-lover who wants to do it right the first time?

Also, if...I mean WHEN bonding is achieved, could the bond be broken when the new female goes for her spay?

The bond could be broken when the female goes for her spay. I had my trio together for a week, and took the girls away from Harvey when they were recovering from their spay, and I bonded all over again on neutral ground, to be on the safe side. It was easier than the first time but I wouldn't have been able to just put them back together.

The pen I use for bonding is 3ft x 3ft which has always done quite well.

I personally would just put them separate until you bond them, not keep them side by side. When I first got Harvey and Zero I put them in a run split in half, and they groomed through the bars sometimes, and sometimes antagonised each other through the bars. When I tried to bond them they had a massive fight. So I don't really think this works And if you swap their litter over you will probably un-litter train your buns.