I'd suggest you check and see if the 3rd SCV kit and loader function kit are still available from Deere. You'll have to go through sales for these as they are whole goods. If they don't have them in stock, make sure the salesman uses his computer to search other dealer's stock nationwide. Otherwise check eBay.
You already have the third spool as it's built into the loader's SCV valve body.
For someone who isn't savvy with hydraulics, this is a quick and easy way to do it, and if the kits are still available, they aren't a horrendous expense. If you don't know what you're doing, home-brewing a setup could get pricey.