If you want to take as much advantage as you can with out having to build a sealed room or "CGE", use a fan dedicated only to ventilation and scrubbing of the air with carbon filter, place it on a environmental controller which will allow you to set the fan on/off for certain parameters ie; humidity gets to high/low, temperature in tent/grow box gets to high/low.
So many variables go into growing good cannabis and environment is the most important along with genetics and skill level/knowledge.
When using oassive intake, twice the area for the intake vs exhaust. Tents have adequate intake area which can be used passivley or actively (intake fan). If using an active, you want the intake fan to be smaller than the exhuast fan to maintain a negative pressure environment. 500cfm may sound like a lot of fan power for a 3x3 but what so many fail to take into consideration is the ambient temperature which is the temperature of the room that the tent sits in and draws it's fresh cool air from. If you can't control that room then best believe with all those olants and all the light and fans working, the temperature in that tent is going to go up considerably. Always best to use an air cooled light (if running h.i.d.'s) with it's own dedicated inline exhuast fan on it's own ducting...should look like
Fresh air from room > Intake ducting > Inline Fan > Ducting > Air Cooled Hood/Light > Ducting > Attic/Another Room...exhuast it anywhere but back into the room it sits in, otherwise you'll be raising the temperature and humidity of the room the tent sits in, therefore rasing the temp and humidity in the tent and so on.
Once attaching a proper sized carbon filter to the fan, as well as any ducting, the fans cfm rating will drop by a good 25% to 30%+ and if you have any 90° or any real bends in your ducting, your fans cfm ratung will be cut by 50%++ so straight a run as possible. Also, use oversized insulated ducting. The larger than needed diameter won't impeded airflow as badly meaning better performance from your fan, as well as cutting diwn on the noise from the air "wooshing" around in the ducting and at it's exit, and because it is insulated you don't have to worry about radiant heat transferring from the ducting back into the ambient room/tent.
I've got some reading material for you in a moment but let's start here...
1) Size and highest temperature and lowest temperature of room you plan to place your 3x3 tent in
2) Wattage of grow light and type (L.E.D./H.I.D./CMH/Fluoro etc.)
3) Max and Min desired temperarures in tent with lights on and off. Tdiff.
