3" raw water seacock question

Assuming it is a tapered plug type valve like in the photo ...

Grease it when it is open. If you close it you will pump grease into the valve port itself.

To get any grease in you will most likely have to back off the retainer nut and lock nut on the small end (opposite the handle) and knock the tapered plug back a tiny bit. Grease that side then rotate the taper plug 180 degrees and add grease to that side. Snug up the adjusting nut while checking the valve for smooth but solid operation then tighten the lock nut and recheck operation.

The size of the zerk required is a greater mystery than life itself.

The plug you are speaking of on the seacocks on your Cabo are for winterizing purposes to let any water out of the body, not for greasing. I guess you could pump grease in there with the valve open. Not a lot of water will flow out of there when you take the nut out......A coffee pot stream......make sure to use a bronze zirk fitting and I belive it's 1/4" Pipe thread.