Before moving forward if i was you i would post the exact same post in the plugin support forum here - http://wordpress.org/support/plugin/wordpress-mu-domain-mapping - and ask for any advice and tips before moving forward.
This thread was in that forum. I closed the other as a duplicate.
For the original poster. You can do what you want with 3 single sites. You can use https://wordpress.org/plugins/members/ to create the roles you need.
If you knew that you would be adding divisions in the future it might be worth your time to convert to multisite and use DM but if you are only going to have 3 sites it ins't going to save you time/effort.