The Security Assist wizard only allows you to select the One Table.
Why is it that you are using 3 separate user tables, for the site?
I'm hard pressed to think if a scenario where this would be needed.
In most cases only user table would be needed, you could use user level authentication to grant different access rights to different users.
There is a User Level Authentication tutorial in the Archived Documentation section of the security assist support page, it was written for SA1, but the concept is the same in SA2.