def mask_3rd(s):
    if len(s) < 2: return s
    s2 = s.replace(s[2], "*")
    return s2[0] +s2[1] +s[2] +s2[3:]

print(mask_3rd("logging"))
print(mask_3rd("apple"))
print(mask_3rd("orange"))
print(mask_3rd("gooooogle"))
print(mask_3rd("i"))