Return sum of three values, excluding values within a specified range
I may be off the mark here as I didn't test my code (no access to a Python environment), but this is the way I'd 'start' to work at it.

def no_teen_sum(a, b, c):
    return fix_teen(a) + fix_teen(b) + fix_teen(c)

def fix_teen(n):
    if n in [13, 14, 17, 18, 19]:
        return 0
    else:
        return n


Quite simple.