First of all you are going to want to have the new chins in quarantine for a month before introducing them to the current chin. That means not only a separate cage but a separate room from your current chin to make sure they aren't carrying any illness that can effect your current one as well as allow them to settle in without having to deal with the possible stress of another chin around. It also gives you a chance to get to know the new guys, each chin is different so what they normally eat, normal activity level, etc.
I assume your current chin is a male as well as the two new ones? If so then yes you could have 3 in a double FN cage, I assume you mean like 2 in one unit and one in the other? One unit is big enough for two, but does give them less room to get away from each other, even bonded pairs need some alone time sometimes. Also although it is big enough, bigger is always better for chins, one unit is the minimum size for 1-2 chins. I wouldn't advise trying to put all three together though, you could very well end up needing 3 cages/units total if they decide to all hate each other. It's not uncommon for a bonded pair to decided they don't even like each other anymore when trying to add a third. They are your chins though, so if you want to try some people have had success with bonding 3, so it's not guaranteed hopeless. More commonly when I've seen 3 chins together it's mother and daughters or father and sons, or same sex litter mates. Age helps to some extent sometimes (like bonding kits together), but just because he is young does not automatically mean they will get along. Personality plays a much larger part in terms of if they will get along or not then age.
If your current one is female however and you mean you want them all together, then I would strongly advise against it, males can breed as young as 8 weeks. Even if you got him to bred with, you'd hopefully want to make sure he is even going to be breeding quality which you can't tell that young.
Also just so you know if he is that young, chins under around 4-6 months (most are big enough by 4 months though) can actually escape a FN cage, they can get through the bars or worse, get stuck in the bars. Another thing to consider if your current chin is female and the new ones are male the males might fight when the female is in heat. In that case you might have to split the two up or move them to another room since chins can and sometimes do fight to the death.
You always want to have on hand as many cages or cage units as you do chins, even if the extra cage is just a small temporary cage, just in case they do need to be split up for any reason (sick, injured, fighting).
