First take the new machine and add it into the cluster, just like you did when adding the second machine to the cluster.
Now edit the resource groups for the two SQL instances and tell them that they can use the third machine.
Then start the SQL installer. When you go through the installer there should be a repair option. In there is should ask you what you want to do with one of the options being something to the effect of add a node to the cluster. It will then push the SQL files and services to the new machine. You will then need to run SP2 (or what ever post SP2 hot fix you are on) to all three nodes in the cluster.
Then repeat with the second instance. Fail the instances to all three nodes to ensure that everything is working correctly.