A number a is multiplied by a number minus b which is equal to three times a number a times a number.
I got ax-ab=3a(3x) am I waaaay off in setting up the problem?

"A number a is multiplied by a number minus b" - so you have a(x-b), which is the same as ax-ab just as you have for the left hand side.

"which is equal to three times a number a" - that would be 3a

"times a number." If you assume this unknown number is the same as the prevuious undefined number that would be x. and so the right hand side is: 3ax

However, given the vagueness of the wording there is no evidence that the undefind number on the left hand side is the same as the undefined number on the right hand side, so it is better to use the unknown 'y' on the right hand side instead of x, to show that it may be different. So you would have:

a(x-b) = 3ay