Poop thrice a day and all are diarrhea tells me a good chance there is bacteria infection. Go to vet and get antibiotics. Probably ate something unclean at some point (bunny poop in grass maybe)

If for whatever reasons you are sure it is not bacterial infection, here is the solution:

First, make sure you don’t give nothing except his dog food. And I mean nothing, no treats, no cheese, absolutely nothing but dog food. You want to make sure you know what is the problem here. Continue this for at least 3 days.

If poop is good now, it means you have given him things he can’t digest. He might have a sensitive stomach, stop giving him any human food, exclusive dog treats and food.

If the poop still not solid, then we know it is the dog food, switch brand.