If even after 3 months none of the dogs are showing any symptoms then there is  less chance that they could get rabies because as per WHO even though the incubation period could vary from 1 week to 1 year but it is typically 2-3 months Also it's very rare that a rabid dog didn't bite because whether dumb or Furious form a rabid dog can be identified clearly with drooling saliva nervous symptoms and a tendency to bite even inanimate objects so there is very less chances that the dog who died of unknown causes had rabies

In this case, we can only speculate, but we can't tell for sure. Since we don't know the exact cause, if he had any signs of rabies before death (like salivation, nervous signs etc) then there is a chance of rabies. If not, then no, it could be some other reason than rabies disease.