If you've ever used some of those ebay enclosures with non-threaded bosses for mounting circuit boards, here's something you didn't know you needed - self-tapping standoffs. I don't know how available they are or if they are available from other sources but these could be really handy.
Oh, but it looks like they're not available with a 3mm thread. Bummer.
You can mount your PCB to the bosses directly if you don't need the extra height under the PCB. I'm sure you know this but you made it sound like standoffs are required to use the enclosure. Other options are to use readily available unthreaded spacers and longer screws or in some cases, invert the enclosure, using the bottom as the top.
I know that, but sometimes you need to elevate a board or tapped metal threads are desirable for a board that may be removed frequently. I like to position boards so that I only need to cut a notch at the edge of the enclosure for connectors and switches.
I recently threaded bosses in a number of enclosures using a steel 3mm screw, and threaded plastic spacers in. It worked reasonably well. A bottoming tap would have worked better, but I don't have one.
Tapping plastic to such a fine pitch will not have much strength. What keeps you from using the M3 tapping standoffs? The assertion of your original post was false and only starts to make any sense with the additional context of your second post.
You seem(ed) not to realize that your use of the enclosure differs from typical in two respects, mounting the PCB higher than usual and desiring durable female threads not included in the design. Of course, it's been thoroughly covered now.