We have: A=3x^2-4x^3+2x-5,B=2x^3-2x-9,C=3x^3-2x^2-15.
"Adding together" the three expressions, we get
A+B-C = (3x^2-4x^3+2x-5) + (2x^3-2x-9) - (3x^3-2x^2-15)
A+B-C + (-4x^3 +2x^3 - 3x^3) + (3x^2 + 2x^2) + (2x - 2x) + (-5 -9 + 15)
A+B-C = -5x^3 + 5x^2 + 1
So the degree of the sum is 3 (the greatest power of x)