You are bringing up two separate issues. I'd like to handle one at a time. If you have an issue with your login page and not being able to use a shared page for login to the three tables, then I'd need to see the three login pages to try to determine the difference.
For the original issue, I think you can just create a new rule from Webassist->Security Assist->Manage Site Access->Access Rules Manager
Create a rule "Logged in to exec or members" with the conditions:
1) ALLOW if $_SESSION['SecurityAssist_Exec_id'] != ""
2) ALLOW if $_SESSION['SecurityAssist_Member_id'] != ""
It looks like your rule for "Logged in to members" should allow access to all groups since the rule is:
2) ALLOW if $_SESSION['SecurityAssist_FirstName'] != ""
And all three of your logins save that session variable. So that rule should allow access to any logged in user as it stands.