The full sequence from 3 to 21 is...
3, 5, 7, 9, 11, 13, 15, 17, 19, 21
Now first add 3 and 5 to get 8. Then 8 and 7 to get 15. After, add 15 and 9 to get 24 and furthermore.
(24+11=35 --> 35+13=48 --> 48+15=63 --> 63+17=80 --> 80+19=99 --> 99+21=120)
So I believe the sum of the sequence is 120
I hope this helps!
