SAN configuration for Exchange 2010 databases Best Practice

Building a new Exchange 2010 environment.  What is best practice for carving space for databases?  I am looking to:

Create 3 databases on a Mailbox server (of course it will be in a two maybe three server DAG)  Each database will have it's own LUN and each LUN will have it's own disk group. 

or

Each database will have it's own LUN, but all three LUNs will be in a single disk group

or

All 3 databases will be in a single LUN in a single disk group.

Comments wanted please

Thank you

Richard