Since TO3000 itself does not feature password-protected access to the program, there are currently 2 ways to do this:

1. Upgrade from TO3000 to Projetex, which features password-protected user accounts.

You can purchase the upgrade here: https://www.to3000.com/order/upgrades-from-previous-versions

You can download a free 30-day trial here: https://www.projetex.com/translation-project-management-software-free-download

2. Use the built-in functionality of Windows or 3-rd party software to protect the database and its backups.

The following page offers a rundown of various methods of protection: https://www.howtogeek.com/105633/how-to-create-a-password-protected-folder-without-any-extra-software/