I suspect that after 3 years the piston rings have rusted to the cylinder walls. More than likely, there's nothing wrong with the electrical starting system (battery, starter, solenoid, etc.). It's just not powerful enough to break the pistons free.
I can think of only one inexpensive and simple but nonprofessional approach. I did it 45 years ago when I inherited a 1950 Willys that had not been run for several years. I pulled all of the spark plugs and squirted rust solvent on top of the pistons thru the plug openings. I let it set for a few days and then sprayed some more in. After about a week I was able to rock the vehicle back and forth in gear and the engine moved slightly. Then what? You guessed it! More solvent and rocking. Eventually, the engine turned over with the starter. As soon as you get it to turn, change the oil and filter!
The other problem is that there is not going to be any oil on the bearing surfaces. If you start the engine once you get it to turn it will damage the bearings and journals. You need to pressurize the oil galley before starting the engine. You may be able to rent something to do that or rig something up on your own.
By the way, I drove my Willys for four years while going to college and never did anything to the engine internals. I sold it to a friend when I entered the Air Force and he drove it for several years until the clutch went out. We never had any engine trouble.