from functools import reduce


def my_atrangi_addition(mynumbers: list)->int:
    """
    This function add every 3rd number in the list i.e. at index 2,5,8,...

    # Input:
        mynumber: list, This is a list of numbers provided by theuser
    
    # Returns:
        int: It returns the calculated sum of the integers
    
    # Functionality:
        This function takes in list, filters out the list containing all third elements 
        and then it is added

    """
    result = reduce(lambda x,y: x + y,mynumbers[2::3])
    return result