I tried doing AAB often after the


, but sometimes they change stance into the one where he ducks so my AA misses and he already hit me. So i always make sure the second

hits.
Then you're not having trouble


,

. You're having trouble with the SCH mixup. UnseenWombat is correct, AAB or A:6 can beat out [3]B, B, but the opponent won't keep doing that string if you keep punishing it. They will use other moves from the stance such as SCH K (quick knees), SCH A (horizontal high stepkill), or the change to BH (the stance where it looks like Sieggy is chilling) where he can crouch your highs.
Essentially you're going to need to read your opponent and adapt. Take note of which move the opponent goes for regularly after [3]B (e.g. SCH B), then apply an appropriate response.
Possible responses to [3]B
Sieggy: SCH B = Natsu: AAB or A:6
Sieggy: SCH A = Natsu: Distorted Breeze (2B+K) or any TC move
Sieggy: SCH K = Natsu: Distorted Breeze (2B+K) or any SS move
Sieggy: BH = Natsu: A+B (perform after the BH GI effect though)
Technically you could nullify most of the mix-up by using 22A, though the Sieg player could delay their response in SCH in order to realign themselves to Natsu. However, you did say that the range of 22A is "too short". I'm guessing that the Sieg player is using [3]B at tip range? At range, try sidestepping to force the opponent to use horizontals such as SCH A.
The greatest challenge with Sieggy is dealing with his keepout and his stance mix-ups. Try to stay close and run in whenever you get a chance (such as after a blocked attack) so that you can have an opportunity to interrupt most of his mix-ups.