The Fastest Horse - got stuck

Hi guys!

Got stuck with lists, which contain nested lists with this code:

import re

def fastest_horse(horses: list) -> int:
    int_horses = []
    for item in horses:
        for sub_el in item:
            indigits = re.sub('[^0-9]','', sub_el) 
            print(indigits)
            smallest_time = int(indigits)
            print(smallest_time)
            int_horses.append(smallest_time)
            print(int_horses)

            a = int_horses.index(min(int_horses)) + 1
            print(a)
            return a

Where I should move to make it work correctly? For example, this assert assert fastest_horse([['1:13', '1:26', '1:11'], ['1:10', '1:18', '1:14'], ['1:20', '1:23', '1:15']]) == 3 returns 1 insead of 3