a(n) = a(n-1) + a(n-2) + a(n-3), with a(0) = 1, a(1) = a(2) = 7.

Terms

a(0) =1a(1) =7a(2) =7a(3) =15a(4) =29a(5) =51a(6) =95a(7) =175a(8) =321a(9) =591a(10) =1087a(11) =1999a(12) =3677a(13) =6763a(14) =12439a(15) =22879a(16) =42081a(17) =77399a(18) =142359a(19) =261839a(20) =481597a(21) =885795a(22) =1629231a(23) =2996623a(24) =5511649a(25) =10137503a(26) =18645775a(27) =34294927a(28) =63078205a(29) =116018907