It all depends on how serious you are about adding a 3rd grate. Me, I added it by splitting the difference between top and bottom. Let's see if the site has changed how we can upload pictures.
As you can see, my method is rather permanent, but that's the way I float. When not in use, I close up the holes with stainless 1/4-20 nut, washer and bolt.
A caution if you will: See the eyelet in the upper right? That's for the temp and pit probes, obviously. Those particular eyelets are made from a zinc coated, brass like finished lamp eyelet tube and have been changed to brass fittings. They're not cheap BUT, I feel better about it.
Rods in place. Note, these are stainless steel rods, for obvious reasons. BTW, they're not allthread.
And this is the shelf, in place.
Just another option for you to consider. Welcome to the darkside. Like others here, I've made many modifications to my WSM. Perhaps more than most, but...........