If my Data, Log and Tempdb drives are on the same LUN, are they using 1 physical drive?
To expand on Aaron's comment of:


  A LUN on a SAN is usually made up of a number of drives. Your data
  will be striped across all of them in some configuration, e.g. RAID 5
  or RAID 10. You'll have to ask your SAN administrator for more
  details; we can't possibly tell you about your physical
  implementation.


Each SAN implements this differently. For example, some can define exact RAID groups, type, etc. Some you just put disks in and they create their own internal sets and configuration - you just tell it how big of a LUN you want and where to zone it. Some SANs will automatically tier your data and you may be on many different sets of drives. Others will yet again operate completely in cache and only flush to disk like a checkpoint operation would in SQL Server.

This also doesn't take into account anything in the fabric that may be acting as yet another intermediate tier such as SVCs.

Really, the question you should be asking is, "Is one LUN allowing me the throughput I require in order to properly run this instance of SQL Server and all of the associated databases."

If there is only one LUN, most drivers for the HBAs won't multipath. This may or may not matter depending on your fabric - but most of the time it helps. Additionally we don't know the queue depth of the HBAs (though not all implementations have such a configurable item) and don't know the typical profile of IO for that server.

Just some extra items to think about :)