How many 3 letter words possible with the word Abacus?

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I am having difficulty solving this. I know that 6 letter words possible with abacus is (6! / 2! ). But my doubt is what happens when we have to choose 3 letters from abacus and how will we handle the repeating of the double a.

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Hint:

  1. How many three-letter words can you make with no $a$'s?
  2. How many with exactly one $a$?
  3. How many with both $a$'s?

These are disjoint sets so you can add them up.