Yes you would build out 3 seperate VM's.
One for each roll. nothing to stop you from keeping 2 rolls on the same host, but this way you can MOVE the rolls around based on performance/needs.
As far as storage goes, Virtualizing realing doesn't change the best practice recommendations from microsoft for sizing of your exchange environment.
you would always want your datastores on dedicated spindels. if you are breaking up your mailboxes into different datastore groups, just go with best practices.
one of the nice things about vSphere is you can basicly virtulize your storage. you dont have to dedicated so many spindels to just one server. you can build out a file system and put some very LOW i/o utility servers on the same luns as your exchange and they are not going to impact your end users experience. now other people here will disagree but most of them are storage vendors with 300-400 spindels in thier SANS. i only have 60... so sometimes you just have to share =D.
I would avoid RDM's unless you just have MASSIVE disk i/o needs. RDM's aren't bad, just makes things a little more complicated.
again My Exchange 2003 banged the hell out of my VSphere implementation, and exch 2010 is barely breathing. With outlook 2010 deployed, so much of the mailbox activity happens in the local OST.
The real key will be disk i/o just make sure you dedicate enough spindels to handel your i/o needs and you should be fine.
