Well if it's a 3/4" screw and one part is 3/4", then the "other two" parts are zero. I'm guessing you meant one part is 3/16 of an inch, rather than 3/4 because it's the only way to get 9/16 as the answer for the sum of the other two parts:
3/4" - 3/16" = 12/16" - 3/16" = 9/16"
You need to have a common denominator to add or subtract fractions. The least common denominator of 3/4 and 3/16 is 16, so 3/4 = (3/4)*(4/4) = 12/16.
