Hydraulic pump has expired or internal leak within the lift cylinder are the most common culprits, other issues are broken internal linkage, there's also a spring in there for the draft control that has been known to break and or fall off, this will render the 3 point non opp, once you've determined you have proper hydraulic pressure and the oil is topped off, Next step will be removing the top cover, Its not difficult but if your not mechanically inclined you probably shouldn't attempt it, if so replacing the cylinder piston seal is no big deal, which you should do if your in there and discover its the spring anyway, its just a cheap rubber o ring and is famous for failure, so might as well replace it while your in there replacing the spring or any other broken parts, There could be a few other things but those are the most common,

You move the lever to raise or lower and the arms do nothing. The hydraulic fluid level is fine and I have changed the hydraulic filter. The linkage appears to be fine. Is there an internal valve that could be bad?

Will not function in what way? Won't go down? look under the seat for a hand valve and unscrew it a turn or two.

fluids fine and it just drops or? check the linkage first, i believe most larger tractors the 3 point is electronic these days.