Concept: The question tests on the basics of Counting/Combination
Solution:
Since all the letters are distinct, another method would be to choose 3 letters out of 6 in 6C3 ways and then arrange them in 3! ways.
6C3 = 6 * 5 * 4 / (3 * 2 * 1) = 20
Total ways = 20 * 6 = 120
Option E