a(n) = 20160*(3*n)!/(n!*(n+3)!^2).

Terms

a(0) =560a(1) =210a(2) =504a(3) =2352a(4) =15840a(5) =135135a(6) =1361360a(7) =15519504a(8) =194699232a(9) =2636552100a(10) =38003792400a(11) =577037174400