The th-order nonlinearity of Boolean function plays a central role against several known attacks on stream and block ciphers. Because of the fact that its maximum equals the covering radius of the th-order Reed-Muller code, it also plays an important role in coding theory. The computation of exact value or high lower bound on the th-order nonlinearity of a Boolean function is very complicated problem, especially when > 1. This paper is concerned with the computation of the lower bounds for third-order nonlinearities of two classes of Boolean functions of the form Tr 1 ( ) for all ∈ F 2 , ∈ F * 2 , where (a) = 2 + 2 + 2 + 1, where , , and are integers such that > > ≥ 1 and > 2 , and (b) = 2 3ℓ + 2 2ℓ + 2 ℓ + 1, where ℓ is a positive integer such that gcd(ℓ, n) = 1 and > 6.