Modularized program to find circumference, diameter, and area from a circle's radius
Many little comments here to make things simpler,shorter,more pythonic :


PEP 0008 gives some coding guidelines for python. Among other things, it gives a naming convention. Also, you don't need the "my" prefix everywhere.
You can use math.pi instead of repeating our own definition of pi everywhere. (Isn't it the reason why your import math in the first place)
Most of your function just need to return a value and updating the parameter probably shouldn't be their responsability.
Most of the variables are not really required.
there's no point to define a main function in your case.


After taking these comments into accounts, your code looks like this :

import math

def get_radius_from_user():
    return float(input("Enter your radius: "))

def get_diameter(radius):
    return radius * 2

def get_circumference(radius):
    return 2 * math.pi * radius

def get_area(radius):
    return math.pi * radius * radius

def show_results(radius):
    print("The Diameter is",get_diameter(radius))
    print("The Circumference is",get_circumference(radius))
    print("The Area is",get_area(radius))

show_results(get_radius_from_user())