A076991 a(1) = 1, a(2) = 2, a(n) = smallest positive integer not occurring earlier such that the sum of three successive terms is a square.

1, 2, 6, 8, 11, 17, 21, 26, 34, 4, 43, 53, 25, 3, 36, 10, 18, 72, 31, 41, 9, 14, 13, 22, 29, 30, 5, 46, 49, 74, 73, 78, 45, 102, 109, 113, 67, 16, 38, 27, 35, 19, 90, 12, 42, 115, 39, 15, 142, 68, 79, 177, 33, 114, 214, 156, 71, 62, 63, 44, 37, 40, 23, 58, 88, 50, 87, 7, 75