My 3 years old daughter has a reddish color by her private parts and inside. what coup it be?

Infection or irritat: It could be caused by a yeast or bacterial infection or a local irritation. Hard to tell without seeing the rash.

Other causes of : vulvar irritation are bubble baths, long baths in soapy water, silk undies or bathing suits that retain moisture, wiping back to front after using toilet &, if overweight, post-voiding urine retention. Bathe her in 6" of water + 1 cup of vinegar, help her wipe front to back with single-use, unscented wet wipes & use cotton panties. Her Pediatrician will prescribe specific treatment after exam.