how to write a python program that can tell whether the numbers entered can form a triangle or not?

i was writing the following code:

def form_triangle(num1,num2,num3):

    success="Triangle can be formed"
    failure="Triangle can't be formed"

    if(num1 < num2 + num3):
        if(num2 < num1 + num3):
            if(num3 < num1 + num2):
                return success
        
    else:
        return failure


num1=3
num2=3
num3=5
result = form_triangle(num1, num2, num3)  
print(result)

but the problem is this code is unable to pass all the test cases. for instance, if num1, num2, num3 have values 1, 2, 3 respectively, then the expected output should be N/A but my output is None. so, what changes should i have to made in my code to have the expected output.