I have a third degree burn on my leg that is untreated. I've kept it clean and aired out. It is now thickly scabbed over, is that ok?

Burn. Apparently it is ok, as long as there is no pus and no surrounding erythema, but if you see pus , you develop fever, pain, increasing redness around the scar , you will need to see a doctor.
Maybe not OK. If you truly have a 3rd-degree, or full-thickness, burn, unless it is very small or narrow & can heal in from the sides, it will need to be skin grafted to fully heal it. Partial-thickness (i.e., 2nd-degree) burns will heal in 1 to 3 weeks. If your wound is not fully healed after 3 weeks, you should see a general, plastic, or burn surgeon.