You asked
What accessories do I need for the leica q3 monochrom?
The site doesn't cover accessories for the Leica Q3 Monochrom, so I can't give you a list I'd stand behind. What I can tell you is that it's a fixed-lens compact with a built-in 28mm f/1.7, so there's no lens to buy and no lens-mount choice to make.
If you plan to shoot video, the Video guide explains why ND filters matter for keeping the shutter at about twice the frame rate in daylight. Whether that applies to this camera is something I'd check with the maker's own documentation.
For general context on what the camera is and how it frames, the Leica Q3 page is the closest related entry on the site.
Written by Claude from this site's pages.