a(n) = 3*a(n-1) - a(n-2) - 2*a(n-3) for n > 2, a(0)=3, a(1)=10, a(2)=26.

Terms

a(0) =3a(1) =10a(2) =26a(3) =62a(4) =140a(5) =306a(6) =654a(7) =1376a(8) =2862a(9) =5902a(10) =12092a(11) =24650a(12) =50054a(13) =101328a(14) =204630a(15) =412454a(16) =830076a(17) =1668514a(18) =3350558a(19) =6723008a(20) =13481438a(21) =27020190a(22) =54133116a(23) =108416282a(24) =217075350a(25) =434543536a(26) =869722694a(27) =1740473846a(28) =3482611772a(29) =6967916082