Chinchillas are very social, as a rule ... it may just take time to get them use to each other (
Very closely supervised playtimes). You may want to try putting both cages on the floor. Let them out to play. The first to cause a problem goes in the small cage for about 10 minutes, for time out, while the other two play. If the next one chases the baby, he can be put in the small cage, too, along with the other one (if his 10 minutes are not up). Then let the kit play with you and then play in the big cage before you let the two older ones back out. {This will also get all three of them familiar with each other's scents, because they will leave that behind when they play/stay in the other one's cage. Chinchillas are very smart ... You would be surprised on how fast they learn that if they are not nice to the baby, they get put in the small cage for a 10 minute time out.
This method usually works, BUT, as usual, there are exceptions to every rule. It could also make them more jealous, which would add to the problem, rather than solving it ... you have to decide if you want to try that or not.
We can give you options that have or have not worked in the past for us, but, when it gets to the nitty gritty of the situation, you have to decide what to do.
Each chinchilla has it's own special personality, and as with humans, some personalities will conflict with others. As a general rule, if they start fighting immediately, it is almost a lost cause ... but there are always a few exceptions.
Jo Ann