I also have three chins together: Maureen (4 1/2 years old), Max (almost 2 years old), and Muad'dib (1 year old, son of the other two). I introduced Max to Maureen when he was 6 weeks old and she was 3. When chinchillas meet there is sniffing and dominance mounting, but as long as there is no blood; I don't worry too much. Other people can help you with techniques. Putting their cages side by side and then later putting them together in a small cage or crate is good.
How old is your older guy? Has he been alone a while? Maureen had been by herself for a year, because she couldn't get along with he other cage partner. She was bored and finally ready to get along with someone. Hopefully, your oldest is ready.
Also, maybe the older one is a little intimidated by two really active little ones. Maybe you can introduce him to them one by one, a little every day. Especially, if your older guy is way past being a juvenile, he may look at those little balls of energy and just be overwelmed.
I'm sure others will chime in. Good luck. Keep us updated.