I think you fail to appreciate that the law directs the CHP and CA DOT to "specify" how the HOV stickers are to be mounted and displayed on vehicles, and they have clearly done so: three stickers in specific locations which are clearly described in the instructions provided with the stickers. Laws passed by the legislature do not have to describe in detail how a state agency implements the laws, they merely have to described an objective and then leave it to the agency to implement it.
If you try to persuade an officer of the law, or a judge, that because the law says "decal" (singular) you only have to display one decal somewhere on your car instead of the three issued to you, I think you will lose that argument. But good luck to you in your efforts to circumvent the published state regulations on HOV stickers, as I don't like them any more than you do: three stickers seems excessive.