How to pick 1 of my 3 formulas in my program?

I just wrote up a program to ask the user to calculate the area, circumference, and the diameter of a circle based on a radius input. However, I'm stumped on how to ask which calculation they want to make between the 3 choices. which is step 1. I have completed 2-4 steps. Any help on this one? (I.e. Only allow them to make one calculation per run of the program) The bolded area is obviously what I tried and failed miserably at....

**Radius = float(input("What type of calculation would you like to 
make? "))
if 1 = "area"
elif: 2 = "circumference"
else: 3 = "diameter"
return("num")**

PI = 3.14
radius = float(input(' Please Enter the radius of a circle: '))
area = PI * radius * radius
circumference = 2 * PI * radius

print(" Area Of a Circle = %.2f" %area)
print(" Circumference Of a Circle = %.2f" %circumference)

import math

circumference = float(input(' Please Enter the Circumference of a 
circle: '))
area = (circumference * circumference)/(4 * math.pi)

print(" Area Of a Circle = %.2f" %area)

import math

diameter = float(input(' Please Enter the Diameter of a circle: '))
area1 = (math.pi/4) * (diameter * diameter)
# diameter = 2 * radius
# radius = diameter/2
radius = diameter / 2
area2 = math.pi * radius * radius

print(" Area of Circle using direct formula = %.2f" %area1);
print(" Area of Circle Using Method 2 = %.2f" %area2)