To solve an equation or inequality, you can add the same value to both sides of the expression. Here, it is convenient to choose that value to be the opposite of the constant (+15) added to y, so that constant is replaced by zero.
y + 15 - 15 < 3 - 15 . . . . . we have added -15 to both sides
y < -12 . . . . . . . . . . . . . . . the result of simplifying. This is your solution.