Is there a way to decrease it? It's way off the charts :)
This might be relevant to MM1-5 too, haven't checked yet.

You can change the mouse sensitivity in the .CONF.
Also, turning off mouse acceleration/smoothing usually works wonders for old DOS games in my experience.

Also MM3 can be played easily without mouse, just from keyboard.
(I did play it like that back in that era, as my PC didn't have mouse at all)
I think the 4+5 is already a tiny bit more cumbersome on keyboard-only, or limited in some way? Can't recall it exactly. But MM3 was fully supported, actually I find it much more convenient on keyboard. Also it has bug which allows you to shoot bows twice per turn by keyboard, which is impossible by mouse clicking on the icon. :))