Just cause a lab still wants to play fetch doesn’t mean there’s still not a pain issue. My first step would be a trip to the vet. Vet’s are very good at analyzing pets and their behavior, as well as our’s. It could possibly have something to do with him still having testicles, but at 3 years old I’d highly doubt there’d be that drastic of a change but you never know. If he doesn’t reside in a house with kids, labs do get more protective as they age. How is he with other dogs? Is there any neighbor kids he knows better than others? That’d be where I’d start. If he’s getting that way with kids he knows well, that’d be ultra concerning. If it’s strange kids, it’s still very concerning. Actual growling and nipping is miles different than barking at strangers. My wife loves dogs and we have a lab and a German wire haired griffon currently, along with 5 kids. She hates it when people say nipping, she’ll correct them and say you mean biting. Personally I do think there’s a difference between nipping and biting, but nipping can turn into biting very quickly. First things first before worst case scenario, take him to the vet. Most are pretty good counselors for helping with tough issues. That’s where I’d start and evaluate it from there. Finding a good professional dog trainer would be my next step after that if the vet can’t help.
Had a great chocolate lab male that was our first dog. This is like 20+ years ago and my wife was walking back into our driveway from taking a walk with our first kid. A neighbors gigantic rottweiler took off after her. Our lab was a big dog but gentle as a feather around kids. He was standing by me in our back yard and took off like a rocket and hit the neighbors dog about 5′ from the stroller. I’d had never seen a dog do anything like that. He T boned that rot and held his own against a mean, big dog. That was not a fun thing to breakup. After that our lab wasn’t so fond of strange dogs, but still gentle as a feather around kids and loved kids. I knew why he didn’t like strange dogs and really didn’t mind. It’s a different situation but changed him drastically. He was also the same dog that wouldn’t let an in-law he knew up our stairs when he came over to work on a furnace issue without us home. He gave him the downstairs, but wouldn’t let him near the bedrooms. Dogs are weird and have some strange personality traits just like humans do.
Is the dog a house dog or in an outside kennel at anytime?