These proceedings contain all the papers that were presented at the Third - ternational Conference on Language and Automata Theory and Applications (LATA 2009), held in Tarragona, Spain, during April 2-8, 2009. The scope of LATA is rather broad, including: algebraic language theory; algorithms on automata and words; automata and logic; automata for s- tem analysis and program veri?cation; automata, concurrency and Petri nets; biomolecular nanotechnology; cellular automata; circuits and networks; c- binatorics on words; computability; computational, descriptional, communi- tion and...