Respuesta :
The required program written in python 3 is as follows :
num = 3
#initialize a variable called num to 3
multiples_of_3 = []
#empty list to store all multiples of 3
while num <=21 :
#while loop checks that the range is not exceeded.
if num%3 == 0:
#multiples of 3 have a remainder of 0, when divided by 3.
multiples_of_3.append(num)
#if the number has a remainder of 0, then add the number of the list of multiples
num+=1
#add 1 to proceed to check the next number
print(multiples_of_3)
#print the list
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