I haven't personally used ona but everyone here talks highly of it.
I went to london drugs yesterday and purchased this thing called an 'Environmental AirSponge', It's all natural and biodegradable yada yada bullshit, but it was 7 bucks.
It's like a little puck that they stick in urinals pretty much, I'd say it removes about 90% of the odor. Maybe 95% but I'm just saying 90 because I've become immune to the smell of weed.
Though I only have it for one single plant, I'm sure it could handle 3 or so (maybe 4) but that's pushing it, Ona comes in way huger containers.