You'd be better off selling it and getting a fixed stock version for several reasons. First, those are rare and people want them for what they are, exactly like they are. They are worth more in that configuration. You could likely sell the top folder and get 1.5x - 2x the cost of a what you are wanting to make. Second, you'll need to change a lot of parts to make it work, Benelli parts aren't cheap. Third, the barrel on those top folders are unique to them. They will work on regular stock as far as I know, but a folding stock won't work on a regular older M3 both because it locks into the rear sight which is in a slightly different position on the folders.
On these older HK folders, they have a self contained recoil system that is kind of wrapped around the bolt. They don't have the recoil rod that the conventional stock needs to mount to. There is no direct-fit collapsible/telescopic stock for these, but I know people have made some fit from another model, seems like it might have been a Nova. It wasn't perfect, but looked good enough.
To the best of my recollection (it's been 25 years since I had the fixed stock version), they have a recoil rod tube that goes down into the stock like the Super Black Eagle, M1, etc. That is what the stock bolts to. Here's a good diagram. Note that the M3 has been re-released and I don't think the new stock and recoil rod works on the older M3's. Here's a good look at how the conventional stock version works: https://www.midwestgunworks.com/benelli-m3/parts.html. The new stock may technically mount to it, but I don't think the fit the contours correctly. The recoil rods on the older ones of the HK era have a different thread pitch if I remember right. There's a few guys that take these apart and mix and match some, hopefully they will come along. I'll tag one that I think has done this on his M3's: @StrangerDanger
Numrich schematic shows both stocks. I kept the one above as it shows only the fixed stock so it makes it a little less busy and confusing. Hopefully this will help. https://www.gunpartscorp.com/gun-manufacturer/benelli/shotguns-benelli/semi-auto-shotguns/m3t-super-90-fold-st
One more link to what looks like a Russian page of the M3 manual, shows there is a different part depending on whether you want a pistol-gripped style stock or straight field stock: https://img.allzip.org/g/60/orig/2210288.jpg