Database design: 3 types of users, separate or one table?

There is also a third choice: put the columns that all users have in common into tbl_users, and create three tables for tbl_admins, tbl_suppliers and tbl_employees joining to tbl_users as 1 to 0..1. You should consider this choice as an alternative when the number of shared columns is significant.

What you need to consider when designing tables is not necessarily what they'll have access to and how that is similar/dissimilar, but rather how the user levels themselves are similar/dissimilar.

For example, if the user types will have the same attributes (name, email, birthdate, etc), then they belong in one table together with a column indicating their privilege level.

This also facilitates changing privilege levels for a user, whereby you can make an ordinary Employee into an Admin, for example, by just updating the record in the user table.

If Suppliers are a different type of object with different attributes than the other two, Suppliers may belong in their own table.

Or, one more thing to consider: You might use a users table that holds only very limited information about users of all three types, and if the types have extended attributes that don't relate well to one another, you can store those in other tables with a foreign key back to the main users table.