a(n) = Sum_{k=0..n} binomial(n,k) * binomial(3*k,n-k).

Terms

a(0) =1a(1) =1a(2) =7a(3) =28a(4) =131a(5) =621a(6) =2986a(7) =14568a(8) =71667a(9) =355159a(10) =1770007a(11) =8862217a(12) =44543786a(13) =224619864a(14) =1135853824a(15) =5757673383a(16) =29247575203a(17) =148847761323a(18) =758776529977