How should I organize my mdf, log and tempdb on a 3 physical disk server
It really depends on your workload but with only two drives you're going to have to make some concessions.  Ideally you want to have tempdb, data and log files all on their own drives.  There's a lot of nasty things going on in tempDB and you don't want that affecting your production databases.

As I said, you should consider your workload, if tempDB is made to work particularly hard or you've got a database that has few write operations then you might find that you're better off putting tempDB on it's own drive and bundling the data and log files together.  In any other situation I'd put tempDB on the same disk as the data files.