trimming masonite 6 panel doors

Tape the cuts to limit splintering. Have a table saw or track saw?

Freud thin blade with 40 teeth or more gave me good results. You can cut close to pencil line and finish with a plane or belt sander.

Is that 3/8 on each side or 3/8 in total?

Check the bottom of the door and see how much thickness the edges have---some of the cheap ones only have 1/2" or so ---removing that much may destroy the door---

If the door is drilled for a lock set--all trimming must be done from the hinge side or the lock set back will be wrong and the door will be ruined---Use a slab door and bore your lock holes after any trimming--

That's what I was thinking.
As cheap as those doors are I'd concider just installing a whole new prehung.

or use a solid core.. no worrying about putting the filler strip back in or the door falling apart over time because the glue joint fails