I would be willing to bet you could buy a factory, bolt-on 3-point for less than the cost of fabricating one. Look at HamiltonBob's website (site sponsor) for lo-boy parts. That's his specialty.
The big problem is that you need to add a hydraulic diverter valve, piping, and separate rear hydraulic cylinder before you add the 3-point hitch arms and stuff. If it were just the arms, it would be a piece of cake and cost nearly nothing. The cost for the hydraulic stuff is typically several hundred $$, and it involves quite a bit of work to get it all piped and hooked up correctly. Add in a couple of hundred $$ for steel, pins, bolts, etc for the hitch, and you can easily spend $500 to build one yourself. That's why I say you're probably better off to just buy a factory one for $400 or $500 and just bolt it on without having to go through all the painstaking work of trying to figure out how to make it work.
1951 Farmall Cub, Cub Cadets 102, 104, 1811, 1864, Simplicity Legacy XL 4x4 Diesel with FEL, 60" mower, 50" Tiller