The goal of all algebra equations is to isolate the variable. In 3 step you should first make sure both variables are on the same side. To do that make sure you use the inverse.
For example:
6a + 9 = 3a + 15
because 3a is positive you're going to want to subtract it.
6a + 6 = 3a + 15
-3a -3a
_______________
3a + 6 = 15
Now it's a 2 step equation and you solve it on from there.