G:=1/product((1-x^(3k-2))*(1-x^(3k-1))^2*(1-x^(3k))^3,k=1..infinity).

Terms

a(0) =1a(1) =1a(2) =3a(3) =6a(4) =10a(5) =18a(6) =33a(7) =50a(8) =85a(9) =135a(10) =206a(11) =319a(12) =488a(13) =714a(14) =1068a(15) =1559a(16) =2241a(17) =3226a(18) =4598a(19) =6448a(20) =9076a(21) =12622a(22) =17415a(23) =23982a(24) =32797a(25) =44496a(26) =60311a(27) =81171a(28) =108698a(29) =145178