switch identity in windows mail (vista)

Well, first of all, you can install all three accounts in the same instance
of WinMail.
Second, WinMail doesn't have identities.
Steve Cochran has written a program to fix that:
WMIDs http://www.oehelp.com/WMIDs/Default.aspx
Windows Mail does not have Identities. However, you can achieve
similar functionality with two minutes of work:

Create additional Inboxes such as Inbox1, Inbox2, Inbox3.
Then create 3 Message Rules:
1. Where the message is from the User1 account
move it to the Inbox1 folder
and stop processing more rules.
2. Where the message is from the User2 account
move it to the Inbox2 folder
and stop processing more rules.
3. Where the message is from the User3 account
move it to the Inbox3 folder
and stop processing more rules.
Hope that helps.