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

Terms

a(0) =0a(1) =0a(2) =1a(3) =3a(4) =6a(5) =12a(6) =23a(7) =43a(8) =80a(9) =148a(10) =273a(11) =503a(12) =926a(13) =1704a(14) =3135a(15) =5767a(16) =10608a(17) =19512a(18) =35889a(19) =66011a(20) =121414a(21) =223316a(22) =410743a(23) =755475a(24) =1389536a(25) =2555756a(26) =4700769a(27) =8646063a(28) =15902590a(29) =29249424