What you need to be doing is apply conditional build tags and generating three outputs. It is then down to your developers to control access via the user's logon. If the topics under say Book 2 and 3 of your TOC are excluded, then the book would not appear. That you can already set up.
RH8 does allow you to have different TOCs but the build will include all topics unless you exclude them via a CBT so you are no further forward.
You could also purchase RoboHelp Server 8 as well as RH8, then you can control user access but that would only work for webhelp. It also increases the cost of upgrading, only you can determine if that is acceptable. You would need RoboHelp Server on each server from which the help will be run.
This question crops up regularly. My own take is to question whether it really is necessary to exclude topics from some users. The system should not let an ordinary user make changes aimed at administrators so does it really matter if they see the help?
See www.grainge.org for RoboHelp and Authoring tips
Just to add one small snippet to Rick's answer - you can use conditional build tags to produce three different outputs with your current version of RoboHelp. There are other reasons to upgrade, but if the multiple TOC feature was the only reason you were considering it, I don't think it applies in your case.