Modern 3-finned bolts in medieval style crossbows?

Not counting a few pistol crossbows, modern crossbows seem to fire 3-finned bolts, with the middle fin facing downwards in a groove, while medieval crossbows used two fin bolts instead. Assuming the prod isn't in the way, is there any reason not to cut a groove into a a medieval style crossbow so i can fire modern bolts?  Has anyone tried this before?

Hello BS, welcome here . There probally is no  reason, what would be wrong with the cock feather straight up. I've tried both ways and can't see a difference unless you'd be hitting something. I've been shooting both regular compounds and crossbow for over 55 years.I still shoot competition . I have even gone so far as to set up my crossbow with a modern arrow rest. Can't really see any advantage except, it's something no one else has. Big deal. There will be others chime in also.I , personally like to shoot regular arrows,30" long . I feel they kill better, in a hunting scheme, much heavier and retain kinetic  energy better.Makes my bow quieter. Plus I have a bazillion of them.

Why not use three vanes at 120 degrees on crossbow bolts with the cock-feather down?  You've already hit on the reason.  Since medieval bows didn't feature center-shot split limbs, the bowyers mounted the prod as high in the stock as possible, to reduce drag and all the weird behavior that comes from having your prod working at an angle to bolt flight.  Though Payne Gallwey has a whole chapter on making bolts with three feathers at 90 degree angles, I can't actually find any examples of such bolts... at least not before say, 1700.  Of course part of the problem lies in the ephemeral nature of feathers.  Bugs eat them over time, so bolts that may once have been feathered are no more, whereas bolts with wood, metal or parchment fletching keep their vanes over time.
As for bolt weight contributing to surer kills in hunting, I have no doubt medieval crossbowmen would agree with you, buit in their case, bolts gained weight more by increased thickness rather than length.  That pays off well when your target has an iron skin.  Thinner, longer bolts would be inconvenient to carry, easier to break and bend.  Short, thick bolts made of suitable wood, like ash, are really-really durable, even when they strike hard targets. 
So by all means, experiment with 3-vaned bolts. They may actually be more accurate, always assuming you get them positioned right.  And don't shoot yourself in the foot!  Geezer.

Post script: As for loading with the cock feather up, I've tried that too.  It works great so long as you don't have to sight directly down the bolt at closer ranges.  The cock feather gets in the way.  If you have sights raised well above the track, that shouldn't be a problem.  Essentially, there's no good reason with a modern, split-limb bow, not to use 3 vanes at 120 degrees with cock-feather down, or with a medieval bow, three vanes at 90 degrees with cockfeather up. Geezer.

You could also use 3 vanes at 120 degrees with cock-feather up, assuming you use an arrow rest, or just bevel the table (bolt-track) downward on the sides to accomodate the angle of the feathers.  The famed Maximilian I and Louis XIII crossbows on display in Vienna (Kunsthistorisches Museum) both have the entire top run, from tail to head, lightly gabled like a low-peaked roof. It looks cool and undoubtedly would reduce string drag on release. Geezer.   Okay, I shut up now.

Geezer; You come across very good. I have to try the gabled approach you mention .Thought I've tried every thing. This will be my next effort. I really never quit building. If not building ,it would be modifing something. My shop hangs full of ideas, some completed , some not, and some abandoned , kept maybe for parts. God, I like this place, greatest gathering humanity ever . For once most all think alike.