Answer:
def prompt_number():
    while True:
        number = int(input("Enter a number: "))
        if number >= 0:
            break
        
    return number
    
def compute_sum(n1, n2, n3):
    total = n1 + n2 + n3
    return total
    
n1 = prompt_number()
n2 = prompt_number()
n3 = prompt_number()
result = compute_sum(n1, n2, n3)
print(result)
Explanation:
Create a function named prompt_number that asks the user to enter a number until a positive number or 0 is entered and returns the number
Create a function named compute_sum that takes three numbers, sums them and returns the sum
Ask the user to enter three numbers, call the prompt_number() three times and assign the values
Calculate the the sum, call the compute_sum and pass the numbers as parameters
Print the result