Use one table. I call the users table their permanent record. I store all kinds of stuff here. Of course their user_name and password, but also their email address, their age, hair color, DOB, name of their grandmother..... Everything associated with them. Easier to do this then link two three table together ever time you need information on one of them.
FYI: google SQL JOINS. There a couple, left joins being the most used. This will help you if you ever need to use two tables. Like in a game and you have 2 or 3 teams. You have to link the team table to the users table.
Evolution - The non-random survival of random variants.