Since you aroused my curiosity, I Emailed the WDFW and asked for a clarification. The rules are as follows:

If the body of water you are fishing has a special rule written as "single pointed barbless hook", then you can fish up to three of these on one line. If you review the regs, you'll see some of these.

If the body of water you are fishing is under "selective gear rules", then you must fish with a "barbless single hook" as described in the definition of "selective gear rules". This means only one hook on the line, only one hook on your plugs.

In other words, "single pointed barbless hook" refers to the number of points on the hook, not the number of hooks. And "barbless single hook" refers specifically to the number of hooks on the line.

Bottom line, if your fishing in a "selective gear" fishery, better not have more than one hook on your line...