Is the third hole in the front, on driver's side, on the side of the valve cover? Is so, I have a valve cover at home just like it, but I don't know what that hole is for either. Mine came with a plug in it, but I don't think I was supposed to run it like. It leaked like hell and I had to turn around within 100 yards to put the old on back on. I hope this helps. John.

'68 C-10 - 250 with Edelbrock 600cfm 4 barrel, Offy intake, Hedman headers and true duals, HEI, MSD 6A box, relocated gas tank
Soon to have: T-5 tranny