a=b=c=0;c(n)=c+n+a;b(n)=b+n+c;a(n)=a+n+b.

Terms

a(0) =0a(1) =3a(2) =15a(3) =55a(4) =183a(5) =588a(6) =1865a(7) =5887a(8) =18550a(9) =58414a(10) =183904a(11) =578936a(12) =1822459a(13) =5736946a(14) =18059358a(15) =56849069a(16) =178955165a(17) =563332829a(18) =1773314909a(19) =5582216334a(20) =17572253459a(21) =55315679765a(22) =174128175040a(23) =548137914348