I can give an explanation for the first two. The last one will take a little longer.

First find your sample size 8 + 7 + 5= 20

A. You want red, green and blue so you multiply 7/20 * 5/20 * 8/20= 280/8000 which divided out is .032

B. Since you can't replace your probability changes but you still multiply 7/20 * 5/19 * 8/18 (you need to subtract one each time in the demoninator since your sample size changes after each marble is chosen.) multiply that out and you get 280/6840 which divides out to be .0408