Why won't my mastiff puppy eat? She is almost three months old and has to be constantly encouraged to eat.?

She is only eating enough to survive. Her ribs are easily seen. I have tried making her dog food. Once I did a chicken and rice combination. She actually ate that well for the first day, but then it is as if she is bored with it and doesn't want it. I have added tuna to get her to eat, but I don't believe that is good for her. I am in a country where veterinary care is limited and so is access to American brand dog foods.