Are you familiar with the settings in Manage | Security? If not, I recommend looking this up in FileMaker help to get some useful background info on it.
You can set up different accounts for each user, each with their own passwords. Each account can be assigned to a different privilege set. What you describe would require 3 privilege sets--one of which is already set up for you.
Your account: make suer that it is assigned the "full access" privilege set. This gives you and only you the level of access needed to make design changes to the database.
User A and C: Give them the same privilege set. As long as it is not full access, and you do not allow editing layouts for their privilege set, they will be barred from making design changes.
User B: give them the third privilege set that limits them to being able to access only the TT table. Chances are good that you may need to provide "read only" access to some other tables if data from those tables is displayed on the layouts this user will use when editing data in the TT table. That will depend on the design of your database.
You will have to go and create user accounts and privilege sets in "File" - "Manage" - "Security".
But before you go clicking buttons in there, please make a backup!
You can create a privilege set and determine what this privilege set can do (what layouts they can access, what records they can view, edit, delete, ...)
And then you can give every user account a certain privilege set.
For more information, please read up on security in the Filemaker Pro 12 User's Guide:
That you can find here:
A litle hint. Don't create any layout access restrictions. Because then people will arrive in a layout and if they have no privileges to see this layout they will just see a grey screen with "access denied" and then they are stuck.
Instead make it so that the records can't be seen. This is more secure, people will still see the layout but every field will say "access denied"
That way people are not stuck, they can still hit a "menu" button (if you have that) to get out of the layout they can't access.
To prevent people from going to a layout they don't have access to you could create a menu (like I always do) with buttons to take them to the different layouts and in these scripts you can add a check like:
If [Get ( AccountPrivilegeSetName ) = Staff]
Show Dialogue box [You have no access to this layout!]
Go To Layout [SuperSecretBankAccountStatusLayout_NotForStaff]
Another hint. Test this well yourself by logging into the file under a user that has a restricted privilege set to see if all works well.
Oh yes, when experimenting with security settings, it's a good idea to first make a backup copy of your file that is not password protected. That way, if changes made to security settings accidentally lock you out of the file, you can discard it and open a new copy of your back up.
Thank you so much guys!