The dog has just had diarrhea for three days, and his mental state is still good. Who knows how to solve it? Should I send a doctor?

It may be a cold, so don't put its nest in a windy place. Then put a mat in its nest, 1 day without eating. If the situation improves obviously the next day, start feeding slowly, starting with semi-liquid food and feeding like boiled dog food.

Dogs can be fed probiotics for gastrointestinal regulation, such as Gooden probiotics.

If the above two aspects do not improve or even worsen after use, send them to a doctor immediately (this is likely to be a minor illness).

I wish your dog a speedy recovery.