You should be in good shape w/ that size grow box.

Check out my first grow, in my sig, so you can see my grow box and what all I went through to get it where it is now. Its kinda what you're thinking, just taller and not as deep.

DIY carbon scrubbers and PC fans are great for ventilation and circulation but I'm sure inline fans and 6-8" ducting will move a whole helluva lot more air.

IMO 3' deep is a waste, I recommend making it roughly 2' deep. Mine is 16" and its kinda tight but 36" might be kinda far to reach in and move stuff around, maybe too much space, IDK.

You might wanna consider scrog'ing too.

Good luck bro :thumbsup: