a(3)= a( 1)+a(2)= 6；

a(4)= a(2)+a(3)= 9；

a(5)=a(3)+a(4)= 15 .

To sum up, the law is like this

a(n)=a(n-2)+a(n- 1).

therefore

a(6)= a(4)+a(5)= 24；

a(7)= a(5)+a(6)= 39；

a(8)=a(6)+a(7)=63 .

A: The order of filling is 3, 3, 6, 9, 15, (24), (39) and (63).