please write the algorithm which is step by step not the code

Write an algorithm that takes 2 integers as input (e.g. a and b) generates a list of numbers between the 2 inputs inclusive and resums the average of all the numbers in the list that are divisible by 3 as an output. Example: when the input integers are [0,15] the output should be 9 (since there are 5 numbers divisible by 3 from 0 to 15,1.e.3,6,9,12,15 )