The basic requirements for setting OpenCore up is *essentially* the same between any cMP. So you can *basically* use any guide for any cMP.
So while it seems this *might* change next month, the guide in the main OpenCore thread (as it currently stands) works fine for a 3,1 (although it is written for 5,1 -- Swap 'Mojave' for 'El Capitan').
The MyBootMgr setup (link in my sig) will always work for a 3,1 since I happen to use a 3,1 myself.