<human>:  greatest common divisor of three numbers.
<bot>: Here is a Python function to the greatest common divisor of three numbers

import math

def gcd(a, b, c):
  """Returns the greatest common divisor of a, b, and c."""
  return (a * b * c) % (a * (b % c))

# Enter the numbers
a = 10
b = 15
c = 20

# Calling the function
print((gcd(a, b, c)))

#Example Output
# 0

#Remember to inspect code for bugs.