# Python 3.6.0

def mask_3rd(strings):
    if len(strings) <= 3:
        return strings

    char = strings[2]
    result = ''
    index = 0
    for s in strings:
        if index != 2 and s == char:
            result = result + '*'
        else:
            result = result + s
        index = index + 1
    return result