It all depends on how seperated you want to keep things.
If thos 3 organisations, whatever they are, have no problem residing on the same server, it can be as simple as adding their domain names to the Default Recipient Policy.
For user account, in AD, make sure only the appropriate users get the respective smtp-address.
The plus: 1 server
Drawback, the admin(s) have access to all of course.
Also, the GAL will by default list ALL addresses.
If that is not an issue, then you don't have to spend more money then needed. Just make sure all organisations are aware to cover yourself.
Marc
If 'something' 'somewhere' gives 'some' error, expect random guesses or no replies at all. Please specify details.
Free Tip: The F1 Key does NOT destroy your PC!