How can i get rid of a sti in my 3 yr olds eye?

Start with heat: Start with hot compresses and gentle cleaning. If not improving see your doctor.

"Stye": Most "styes" are really chalazions involving the oil glands of the lids. The proper treatment is with an antibiotic and steroid combination drop or ointment 4 times a day for 5-7 days with hot soaks also 4 times a day (5 minutes each time is enough). If not gone, then surgery will be needed to remove the cyst. Or if is small, you can live with a small cyst with no problem.

Try warm compresses: If it is a stye, it is not an infection and can be treated with warm compresses twice a day for ten minutes. It should resolve in a week or two. If it doesn't, you should get your child evaluated by an eye care professional.

Stye: See a pediatric ophthalmologist. They will recommend warm/hot compresses (applied carefully), vs Azithromycin eye drops, vs injection of steroid or excision of the stye.