My 5 month old has decreased appetite and passes stools 3 times a day (the usual for her is 1). Stools are normal, not runny. Could it be teething?

Probably not. Teething symptoms are not usually related to appetite or bowel movements. If teething is the issue it may be the texture/feel of the food in the mouth that may keep your child from eating. Try food with different textures, flavors, or temperatures to see if appetite returns. If not then it could be something else and if she/he is not eating enough see the doctor.
Maybe. I can be hard to determine the exact cause of your infants symptoms... It might be related to teething or it might be something else. Encourage fluids and monitor your child temperature.
Could be. Some children do get a low grade fever with teething. If the rest of her behavior is basically normal then just give it more time. It is important to keep her hydrated with fluids. As long as she doesn't stop eating and drinking a day or two of reduced eating is no problem. If other behaviors change you should have her pediatrian check her out just to be sure she doesn't have a cold as well.