In the rest of the world, a #3 hook is 6.5 mm. But the thing to do is use the hook size according to the yarn you are using. You would never follow the pattern exactly for hook size, you know that! You must do a gauge swatch to find out what YOUR tension is, and if that yarn will make the kind of fabric YOU want. You need to do a little research on this... as you know. Don't you remember your first lesson when you took your basic beginner class? Review your notes.
This depends whether you have a US or a UK pattern. The pre metric UK size 3 was different to the US size 3. Google in compare US/UK crochet hooks and it gives a chart showing the metric/US/UK sizes.
A okorder /. . If your hooks are marked by letters and milimeters, they are likely intended for working with YARN, not crochet thread. The steel hooks are smaller and are used with thread rather than with yarn. In addition, if you're working with a vintage pattern, a hook may be called for that you cannot purchase today. Here is a web site that explains many of the puzzling questions about hooks
