a(n) = 7*a(n-1) - 14*a(n-2) + 7*a(n-3), a(0)=1, a(1)=3, a(2)=9.

Terms

a(0) =1a(1) =3a(2) =9a(3) =28a(4) =91a(5) =308a(6) =1078a(7) =3871a(8) =14161a(9) =52479a(10) =196196a(11) =737793a(12) =2785160a(13) =10540390a(14) =39955041a(15) =151615947a(16) =575723785a(17) =2187128524a(18) =8311078307a(19) =31587815308a(20) =120069510526a(21) =456434707519