a(n) = floor(r*a(n-1)) + floor(r*a(n-2)) + floor(r*a(n-3)), where r = 3/2, a(0) = a(1) = a(2) = 1.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =6a(5) =14a(6) =34a(7) =81a(8) =193a(9) =461a(10) =1101a(11) =2631a(12) =6288a(13) =15029a(14) =35921a(15) =85856a(16) =205208a(17) =490477a(18) =1172311a(19) =2801993a(20) =6697170a(21) =16007210a(22) =38259559a(23) =91445908a(24) =218569015a(25) =522411722a(26) =1248639967a(27) =2984431055a(28) =7133224115a(29) =17049442704