You'll probably need calmag unless your base nutrients and (if using) tap water have a bunch.
It'd probably be good to search for info about people doing dry amendments in coco here and on AFN. It seems like his method is hit or miss for a lot of people, though I'm not clear if it's because people are using different specific brands of things than Mr. Canuck (some are hard to find outside Canada) or what. IIRC lots of pH issues? The pH he recommends for the water seems suspiciously high to me. I grow in coco, but use hydro nutrients (Dyna-Gro) and bottom-feeding.
Negative pressure from your exhaust fan may be enough. I have a 2x2 and my 4" fan has been plenty.
Several people have mentioned the Vivosun carbon filters crapping out after a couple months. The cheap Vivosun pH meter probably sucks, but it seems like most cheap pH meters do. (I use the pH test drops in the pH up/down kit, but I only really care about adjusting to 5.8-6.0ish.) I use this one (but the 4" version) and it's worked well.
That light may be inadequate for a 3x3, might want to double check. There's a good lights megathread pinned here with recommendations.
I'm actually upgrading and i got this filter