Disable the Welcome email within CB > Configuration > Registration by emptying the Subject and Body. Now install CB Auto Actions and create 3 email actions similar to the below. You now will have 3 usertype specific welcome emails.
Type: Email
Trigger: onUserActive
Access: Your Specific Usergroup (this is why you need 3)
To:
Subject: Welcome to our site!
Body: Thank you for joining or awesome site!
Subject and Body can of course be anything you like. They also both support substitutions so you can add [username], etc.. to them as needed.