Or it could be The Greek letter Rho which has several uses, and in Greek numerals represents 100.
With the combination of easy as 1, 2, 3 in the clue, I'm more inclined to lean toward my original guess. Abc is as easy as 123. Meaning our missing letters should be placed in their alphabetical order. So instead of Abc being 123, HOR is 123.
The use of scrabble tiles suggests to me that the numerical equivalent of the letters gives the hint toward the number of each letter needed.
So 1 H, 2 Os, and 3 Rs. This is where my dylexia comes in handy again to create the word HORROR.
That's how my mind sees this particular puzzle.