I Gave My Child A Dose Of 15ml Amoxycillan For

I gave my child a dose of 15ml amoxycillan for strep because the pharmacist told me to do that instead of the 5ml 3 times a day. she said do 15ml once a day. but reading the prescription it says 5ml 3 times a day. have i overdosed him? and can i go back to the 5ml 3 times a day. this was the first dose. 5 3/4.