At what temperature do third degree burns occur?

It varies a lot. This depends on the source of the heat, temperature and duration of contact. One reference quotes the following: hot water causes third degree burns… …in 1 second at 156º …in 2 seconds at 149º …in 5 seconds at 140º …in 15 seconds at 133º the very young and elderly are more suceptable.
Complicated Answer. The "degree" of a burn refers to the depth of tissue destruction. A 3rd degree burn involves full thickness injury to the epidermis and dermis. While tissue damage starts at around 44 c (111.2 f), other factors such as length of contact are important. I.E. Higher temperatures can create more damage faster. In addition to heat, chemicals, electricity, and radiation can also cause burns.