This is a very easy to do, you need to first get the third machine all built up, added to the domain. Once that is done add it to the windows cluster. You can't do anything until it is part of the cluster. Then you install SQL using the add a node feature in setup and get it setup. I also recommend you do this for BOTH SQL instances even if you don't necessarily want to have both here. In a pinch it can save a lot of hassle. Also make sure the SAN will let the third node see the disks and I think you have to add them to the third node, but they would be reserved there.

I do want to point out something important with an A/A/P configuration, your cluster mode is REALLY important. Right now with a 2 node cluster I'm sure it is "Node and Disk Majority", this basically means that the Quorum and one node must be available or the entire cluster is down. Think of it this way, for the cluster to be up 50.1% of the nodes/quorum must be available. So in a 2 node cluster you can have a node die but the quorum is still active and the other node is active, that is 67% of the resources. Think about this with three nodes and a quorum, if one node dies you still have 75% of the nodes/quorum, if two nodes die you have 50%, in that case the ENTIRE cluster is down. The only way to do this is to set the cluster mode at "No Majority", this case allows everything but one node to die and the cluster remains running.

CEWII