Question
I added about 2-3 inches of top soil to my lawn without killing the old grass, which was spotty. What will happen if I didn't kill or remove the old grass first?

Answer
Hi John, Many grasses will grow through the soil, but weeds are easy to eliminate.  I'm more concerned with what you've done.  If the 3" of new soil is different than the underlying soil, you may have problems.  It's not deep enough to support grass and watering will be difficult.  You should till it into your subsoil for a uniform texture.  If there are tree roots in the area, you may damage or kill the tree by covering the roots with as little as an inch of soil.
Jim