3^(n-3)*n*(n-1)*(n-2).

Terms

a(0) =0a(1) =0a(2) =0a(3) =6a(4) =72a(5) =540a(6) =3240a(7) =17010a(8) =81648a(9) =367416a(10) =1574640a(11) =6495390a(12) =25981560a(13) =101328084a(14) =386889048a(15) =1450833930a(16) =5356925280a(17) =19514513520a(18) =70252248672a(19) =250273635894a(20) =883318714920