Losing adult teeth as young as 3 years old, is Not normal! Your Chi is too young to start losing adult teeth. It really sounds like he may have Dental Disease, causing him or her to start losing teeth and that needs to be checked out by a vet. You can always call your vets office, tell them that you 3 year old Chihuahua lost a tooth and ask them if it's normal and see what they say. 
