It actually depends on how your mail is setup. If you set all three individually through BIS then when you compose a new email at the top of the new email page is a field that will say "send using." Just change to the email address you want to use.
If you have 3 email accounts all forwarding to one address it's a little harder to do. You need an app like BBReply. Or go ahead and set them up properly in BIS setup if you can.
Options->Advanced Options->Message Services will allow you to set the default. However, if you've replied to an email address in the past it will default to the account you used last on replies and new messages. On new messages, you can scroll into the header to change the account you're sending from.