a(n) = (7*floor(a(n-1)/3)) + (a(n-1) mod 3) with a(1) = 3.

Terms

a(0) =3a(1) =7a(2) =15a(3) =35a(4) =79a(5) =183a(6) =427a(7) =995a(8) =2319a(9) =5411a(10) =12623a(11) =29451a(12) =68719a(13) =160343a(14) =374131a(15) =872971a(16) =2036931a(17) =4752839a(18) =11089955a(19) =25876559a(20) =60378635a(21) =140883479a(22) =328728115a(23) =767032267a(24) =1789741955a(25) =4176064559a(26) =9744150635a(27) =22736351479a(28) =53051486783