The third type of rock is called:
Metamorphic Rock,
This type of rock happens when a Sedimentary and sometime s Igneous rock.An example is when an Igneous rock,granite,is turned into gneiss The rock is buried and put under preacher where other minerals are mixed into it and forms into a metamorphic rock.If it gets below the crust and melts, it will turn into magma or lava when it is on/above earth's surface. Than it Will turn into Igneous rock. The changing from rock to rock is called the rock cycle. From Igneous to Sedimentary to metamorphic and back to Igneous these all are rocks.
One of my favorite websites to go to is
http://www.fi.edu/fellows/payton/rocks/create/index.html
