a(n) = 3*binomial(2n-1,n).

Terms

a(0) =3a(1) =9a(2) =30a(3) =105a(4) =378a(5) =1386a(6) =5148a(7) =19305a(8) =72930a(9) =277134a(10) =1058148a(11) =4056234a(12) =15600900a(13) =60174900a(14) =232676280a(15) =901620585a(16) =3500409330a(17) =13612702950a(18) =53017895700a(19) =206769793230a(20) =807386811660