Boardsort has an ai they've been working on, it's not currently available though. Other ai people have mentioned have been hit or miss on correct identifications.
If you're just sorting into three categories you can probably do it without an app.
Low grade is generally brown on one or both sides and doesn't have any precious metal/high value bearing components. They can have a decent amount of steel or aluminum or plastic on them but there is a tipping point where there's too much junk and it becomes a shred category. Power supply unit boards are a good example of low grade.
Midgrade would likely be low grade except that it has some amount of precious metals and the amount of junk should be generally less than a low grade board or the board is higher quality (not a brown board) with minimal junk but also minimal good stuff.
Everything else should fall under "high grade" which can be broken down into many more categories. If you want a good thread regarding all the material grades check it out
here