full program asks user to input 3 numbers and it will show the sum of them

def add(a, b):
  return a + b

num1 = int(input("Enter first number: "))
num2 = int(input("Enter second number: "))
num3 = int(input("Enter third number: "))

print("The sum of the three numbers is: {}".format(add(add(num1, num2), num3)))