First of all when getting another chin you want to quarantine it in another room for 30 days to make sure it's not sick and give it a chance to settle in. That being said, I don't advise 3 chins together unless it's mother and daughters, father and sons, etc. It's very common for it to not work out, bonded pairs split (either fight or the pair shifts to one bonding with the new guy and not his brother anymore) or you end up with two against one, or everyone hating everyone. It's a very real possibility you will end up needing to house all three separate in the end. I'm not saying that 3 chins together always fails, it works sometimes, but it has a much higher failure rate then bonding 2 together. A better option if you want another chin is to get the chin and keep it separate in it's own cage. Caged in the same room he can talk to the others but wont damage the existing bond. You can try playtime with them all together, but personally I think it's a risk trying to house them all together.
If you are willing to take the risk, I would try just introducing them separately during playtime, let them bond individually so they can't gang up on the new guy. If all goes well after a few weeks or months then you can try with all three together. If that works after a week or so you can try in the cage together, but watch them. You also want to scrub down the cage they are all going in so it smells like a brand new cage and not like any of the chins, and rearrange the cage so it looks different. That way the chins don't immediately see it as their territory since it looks different and doesn't smell like them.
Keep in mind too that just like humans don't like or get along with all humans, all chins don't get along with all other chins. Just because they get along with each other does not mean they will get along with a new chin.