a(n) = 3*binomial(4*n-1, n-1)/(4*n-1).

Terms

a(0) =1a(1) =3a(2) =15a(3) =91a(4) =612a(5) =4389a(6) =32890a(7) =254475a(8) =2017356a(9) =16301164a(10) =133767543a(11) =1111731933a(12) =9338434700a(13) =79155435870a(14) =676196049060