a(n) = a(n-1) + a(n-2) - a(n-3).

Terms

a(0) =18a(1) =45a(2) =69a(3) =96a(4) =120a(5) =147a(6) =171a(7) =198a(8) =222a(9) =249a(10) =273a(11) =300a(12) =324a(13) =351a(14) =375a(15) =402a(16) =426a(17) =453a(18) =477a(19) =504a(20) =528a(21) =555a(22) =579a(23) =606a(24) =630a(25) =657a(26) =681a(27) =708a(28) =732a(29) =759