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

Terms

a(0) =3a(1) =6a(2) =28a(3) =165a(4) =1092a(5) =7752a(6) =57684a(7) =444015a(8) =3506100a(9) =28242984a(10) =231180144a(11) =1917334783a(12) =16077354108a(13) =136074334200