(1)原式= 1 3 (5×6×7-4×5×6)=30,(2)原式= 1 3 ×100×101×102=343400;(3)原式= 1 3 n(n+1)(n+2)= 1 3 n3+n2+ 2n 3 .故答案为 1 3 (5×6×7-4×5×6),30; 1 3 ×100×101×102,343400; 1 3 n(n+1)(n+2), 1 3 n3+n2+ 2n 3 .