My 3yr old stopped eating as he normally did. he eats a couple bites of his breakfast he will eat a bit more at lunch but doesn't want dinner.

Is he growing: At that age I expect a kid to eat well 3-5 days a month and nibble the rest.This follows a reduction in growth rate. If his intake sustains normal activities for age and his growth is stable (checkups), it is likely fine.Try not to be seduced into offering junk food as his intake per day will stop when his internal monitor says to.

This is frustrating : But, very normal. If you look at a growth curve for a 3 year old, it really slows down through this period and is most nice able by the eating patterns you describe. If he is not eating a good mix of foods, a multivitamin might be helpful. Most kids do not need them as ensuring a wide variety of fruits, protein, vegetables in creative ways can help.