If we are observing 3C's, why is it concentration territory? I understand you can never really fully separate concentration/insight, but should I be artificially solidifying stuff here, or tuning into the flickering? Thanks for the help!
JB puts it really well when he says that it's a question of ratio, there's a balance to it that's sort of like tuning in a radio to the station you want to hear or dialing up a good tone on an amp. You know when you find it because it sort of clicks, things begin to line up and you can see what needs to be seen.
Don't solidify things, you'll end up stuck in 4th jhana which, while thoroughly enjoyable, isn't what you want to be doing if you want stream entry. Get that balance between stable concentration and attentive, alert investigation then stay with it. It's easy to drift off or get complacent while you're in Equanimity just because everything seems so natural and effortless, there's also little dropouts which you can mistake for fruitions so staying alert and curious about what's happening will, hopefully, prevent mistaking things or losing focus.
Watch the flickering, see if you can follow it from the moment it arises till the moment it passes, catch what's happening in between the frames of reality as they flicker in and out of existence. This is seeing fundamental impermanence, all observable phenomena including the sense of the observer itself can be seen to flicker in this way, what does that tell you about the observer? If it can be seen then it can't be the things that's observing it, so what's actually observing it?! If it's not the thing which is observing it, and it's a flickering, impermanent sensation then it's not going to satisfy you because it'll just vanish again in a second! Seeing the 3C's clearly is as simple as looking at the way in which all sensate phenomena behaves; it's a conceptual framework which allows you to see the world in a certain way, a way which is incredibly effective at showing you, in the most in-y'er-face-then-booting-you-right-in-the-swingers way, what it is that leads to suffering.
Is that any more useful? Ask away if you have any questions, I can only base this stuff on my own experience but I'm happy to help anytime. Your own experience will show you much, much more anyway. ; )