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

Terms

a(0) =3a(1) =1a(2) =3a(3) =7a(4) =11a(5) =21a(6) =39a(7) =71a(8) =131a(9) =241a(10) =443a(11) =815a(12) =1499a(13) =2757a(14) =5071a(15) =9327a(16) =17155a(17) =31553a(18) =58035a(19) =106743a(20) =196331a(21) =361109a(22) =664183a(23) =1221623a(24) =2246915a(25) =4132721a(26) =7601259a(27) =13980895a(28) =25714875a(29) =47297029