I gotta admit, it is a bit confusing that it says, "one <insert wad here>"
One? You mean I can fit two in there?
All the manuals write it out that way though. It's a holdover from when stacked cards and fiber wads were used. It used to be you would have something like "two 1/8" nitro card, one 1/2" 28ga. fiber, one 1/8" hard fiber"
Instead we use' "one plastic wad".

ETA, not all but many steel shot 3" loads use under shot cushions, usually felt or sometimes cork or whatever. The wads are listed in the order in which they are inserted into the hull. If it says "one Rem SP12, one 1/8' 20ga. felt", then the Rem SP12 gets stuffed in before the 20ga. felt.