How do I fix a stuck toilet tap that hasn't been turned in 3+ years?
Unfortunately there is not going to be any "trick" that is going to make this easy. Corrosion or mineral deposits may have bonded some of the moving parts. There are basically two ways to attack this problem. Either use brute force or attempt to disassemble the valve. Before doing either, be sure to locate and check the operation of the main water shut off that feeds this toilet. You want to be sure you can turn off the water if the jammed valve in question leaks, needs to be disassembled, or removed/replaced.

Personally, I won't mess around with an old valve for long because a new 1/4 turn ball valve replacement is cheap ($4 - $7), and easy to sweat in place with a new feeder/riser tube or flexible Stainless steel hose to the toilet.

Let's try operating your frozen valve. The brute force method is simply using a pair of pump pliers or a medium length monkey wrench on the handle. Get a grip on the thing and try to open it using the leverage and length of the pliers etc. Applying enough force with a long enough handle will either move the valve or break the valve handle. Assuming you are able to get the thing to move,  the next step is to check it for leaks and see if it will close tightly again. Often, they will not close completely as the seats may be damaged or debris is stuck inside.  This is were disassembling the valve comes into play.  By removing the nut from the valve body on the operating shaft, you can pull the whole cartridge out. Once this cartridge is out, you can inspect it, clean out debris, replace bushings or packings etc and reassemble. If this is an old multi turn valve, it is rarely worth the time and energy to fix it. However, it can be done. This is where I refer you to plan 1, replace it with a new 1/4 turn ball valve.