On my 4400 the third SCV ports are ISO 6149 female metric o-ring with M18x1.5 threads. The factory kit uses metric to o-ring face seal adapters which mate to the steel hard lines that come in the kit. If I was doing a homebrew kit using hoses rather than hard line, I'd probably get adapters to go to 3/8" JIC. Avoid pipe thread whenever possible IMO.
967-6-18 | 3/8" JIC Male x M18x1.5 Metric Male: DiscountHydraulicHose.com
Looking back at the OP's pics, it appears his unused 3rd SCV is the outer one. On mine it is the inner, the outer two run the loader. I had to disconnect the loader lines to get a wrench on the inner fittings...bit of a pain and less spacious than OP's pics. Check which way yours is configured.