G.f.: [Sum_{n>=0} x^(n*(n+1)/2) * (1+x)^n ]^3.

Terms

a(0) =1a(1) =3a(2) =6a(3) =10a(4) =18a(5) =27a(6) =37a(7) =54a(8) =81a(9) =106a(10) =132a(11) =180a(12) =245a(13) =306a(14) =381a(15) =493a(16) =612a(17) =729a(18) =910a(19) =1173a(20) =1434a(21) =1662a(22) =1950a(23) =2379a(24) =2925a(25) =3522a(26) =4146a(27) =4831a(28) =5628a(29) =6600