(15-6)-3 = 15 - (6 -3) dose this make sense or nonsense?

No it does not make sense. You misused the parenthesis and the "-" multiplied the variables within the parenthesis and as a result one of them subtracted (15 - 6) by 3 and the other added, which is why the final values are 6 apart. This is not how you use properties in math.
(15 - 6) - 3 = 9 - 3 = 6
15 - (6 - 3) = 15 - 3 = 12

(15 - 6) - 3 = 15 - (6 - 3)

First, let's simplify everything on the left side.

9 - 3 = 15 - (6 - 3)

Now, let's simplify everything on the right side.

9 - 3 = 15 - 3

Then, simplify both sides.

6 = 12

Is 6 = 12?

NO! 6 ≠ 12

Therefore, this equation is not true :)

~Hope I helped!~