a(n) = 3*a(n-1) + 4*a(n-2) + a(n-3), a(0) = 1, a(1) = 1, a(2) = 5.

Terms

a(0) =1a(1) =1a(2) =5a(3) =20a(4) =81a(5) =328a(6) =1328a(7) =5377a(8) =21771a(9) =88149a(10) =356908a(11) =1445091a(12) =5851054a(13) =23690434a(14) =95920609a(15) =388374617a(16) =1572496721a(17) =6366909240a(18) =25779089221a(19) =104377401344a(20) =422615470156