Do you mean he 3bets 9% from the SB vs BTN or 3bets a total of 9% vs BTN?  If the latter I'd recommend getting specific stats for SB vs BTN and BB vs BTN.  

Start with the fact that a 9% 3bet vs BTN, particularly from the SB is very tight so don't worry about being exploited, he isn't trying to exploit you.  

yeah i think i should add specific SB/BB vs BTN stats, i only have "3b by position" and "3b vs position". But thats not my point here, i want to know how much i must defend against an x% 3b. How much do i have to defend vs 15%, how much vs 10 ...

Matt Janda's videos try to deal with this but you can only approximate things unless you have a deal with the other guy that you'll both only raise or fold.  

My first comment does give a beginning of an approach to take.  Don't aim to make this guy's range 0EV, just defend as wide a range as possible where the worst hand does at least as well as ending up with your starting stack minus your open vs the best 9% range he can have versus yours.  This means you have to take postflop play into account and so it's not possible to "solve".

He's risking 7.5BB to win 4BB, so he breaks even if you fold 65% of the time.
Because he'll be making more money post-flop than you (range advantage, initiative), you should be defending more than 35%. The gap between MDF and GTO defense frequencies is nearly impossible to calculate.

If you fold to 60% of 3-bets and calls 40%, you should do fine.

Yes but that's not telling the whole story.  If he's 3betting 0.45% who cares if he's breaking even?  First of all you have to determine whether he's 3betting wide enough that you should care whether he breaks even.  3betting 9% vs a btn open you probably shouldn't.

Also, the 65% would be correct vs an exploitative 3betting frequency if you only 4bet or fold.  If you're calling you have to go wider but there's that "but..." again.