If you leave them alone they will be fine. The key point here is not to let the older one feel foresaken for the newbies.
Pay the old man special attention while the newbies and he get acclimated to each other. Cosmic Catnip is the perfect treat for him as the kittens have not developed and affinity for catnip yet.
They will never be perfect pals but things will work out just fine if he doesn’t feel his “position” with you has been compromised by the kittens.
I have been making sure that Smeagol gets first attention. Food, hellos and making special trips into the bedroom to spend time with him.