Is it ever too late to socialize a Corgi?

Our Corgi is so sweet, but EXTREMELY territorial with not only us as a family, but our home as well. He has bad behavioral habits towards new dogs, and has made it very hard for us to bring him out around other dogs. He will snarl at any unfamiliar dogs, and if I’m not readily paying attention he has snapped at the dog that is wanting to say hello or play.

He is now 3 years old and I’m scared we have lost the window to correct these issues. If not, what advise / training habits could you recommend?