Hi Gina, how old is Gus? If he's a baby he will lose his teeth, but if he's an adult I don't think he should lose them (for no reason). I know what you mean about the 3-in-one, that's a large tooth! It may have been really infected for a while or something and finally fell out? Maybe take Gus to the vet? Unless he's really young then that would be normal, I believe. Keep us posted!