The argument from bulls is that today's selling has been triggered by quarter end flows and window dressing but how confident can anyone be in that assessment? Or that there's a scope for a big bounce in Q4?
Certainly the price action in equities today doesn't fit with what's happening in bonds and FX so there's a reasonable basis for the belief but there's nothing like a 2.7% decline to the worst levels of the year to challenge a thesis.
In any case, the technical picture isn't pretty on the break below the June low. That level was 3636.87 and that will be the level to watch into the close.