Do you actually know what a "3rd-degree burn" is?? It's where you burn the skin so much that it damages the epidermis (surface), dermis (main layer) *and* the subcutaneous fat below. 3rd-degree burns result in blackened, charred skin that can not heal, and typically required large amounts of skin to be removed and replaced with grafts. This kind of burn is the sort of thing that happens to people who get stuck in a burning house for a long time, not people who spill a hot liquid on themselves. Coffee, no matter how hot, will evaporate and cool off long before it can cause that kind of damage. I'm betting the worst you could get from spilled coffee is 2nd-degree, and even that would have to be some pretty damn hot coffee...
And 2nd degree may over time progress to 3rd degree