A couple months ago I introduced a new male into a bonded pair. Here's what I did...it's a plan you can use if you decide to go forth with the addition.
1. Side by side carriers while I did a full cage clean and re-arrange. Bonded pair in one, newbie in the other. This allows them to get familiar with each others scent and each other.
2. After a complete cage clean and re-arrange (Remove the wheel if you have one, you want the cage as open as possible in case you need to intervene.) I moved the cage to to center of the livingroom. This will help in making the bonded pair feel like they're in a new cage/environment. Also, caged animals don't like being exposed on all four sides, it makes them feel uneasy. This will aid in giving everyone the same uneasy feeling. You don't want anyone feeling like it's MY territory and feeling over confident.
3. Once the cage is ready, I placed the newbie in the cage, and then moved the carrier containing the bonded pair in the center of the cage. There were kacks and movement inside the carrier...trying to get to the newbie. After a period of time, the drama lessened. After three hours, they were all sleeping. The newbie was sleeping up against the carrier wall with the bonded pair.
4. If you get to this point, I would put the newbie back in the carrier and take him/her to an unfamiliar room to your chins. Once there, go back and get one of the bonded pair, and place him/her in the carrier with the newbie. (This is called smooshing) I watched them for 10 minutes and had no issues whatsoever. After that, I removed the bonded pair chin and switched him out with the other member of the bonded pair. No Issues. You need to be able to react to this smooshing immediately. It may become violent? They may struggle for dominance? There's a difference between the two scenarios and you just want to let them be if you're not seeing tons of fur flying or biting. They need to work this out, and even if you do a different, longer form of introduction, they will still ultimately go through this.
5. At this time I felt ready to place them all together in the big cage. Before doing so, get a large dustbath ready (This tip came from Mark Miller). One by one, place the chins in starting with the newbie first. Once they're in, quickly place the dustbath in. It's a good way to put their focus on something else right off the bat, and it's fun for them. Do we ever see our chins as calm and content as after a dustbath? Mine are kind of in a zen state afterwards....sometimes sitting in the dustbath in a state of peace. lol
This is a LONG day! So if you decided to go forward with it...make sure you have a long time to do the introduction process...as well as days afterward to keep monitoring their interactions. Being in the middle of the livingroom it's easy to keep an eye on them and hear noises from any agression that MAY develop. Plus I did a lot of walking around the cage...reminding them they were exposed on all sides. It kind of startles them sometimes and makes them more reliant on each other for security.
I hope this helps. It worked beautifully for me! Although I will call out here that even though my introduction went well, I had a hard time adjusting from the mess of two chins, to the mess of three. The mess increased GREATLY, more than I had thought from just adding one more chin. I struggled with it for a while before figuring out how to deal with the added mess on a daily basis. Just something else to ponder...
Good luck on your decision!