advice on getting a 3rd chinchilla

hello people i have been up to a local pet store and have seen a lovely male chin who looks like he would love a new home, i jave 2 male brothers atm and i have a large cage over 6ft long so their would be enough room for another, how easy or how would i go about this? i have seen a few methods online about a cage within the cage and that but as i have 2 chins i dont think that would be best if i had to put the other 2 in the smaller cage thanks