400 watt would be perfect for that size of area. Just take your sq footage (3'X3'=9 sq ft., exclude your height), so 9 sq ft. and multiply by the atleast 5000 lumens/sq. ft. that is ideal and you would need atleast 45,000 lumens. 400 watt hps puts off 50,000-55,000 lumens depending on your bulb. I've got a 400 watt and it's definitely worth it just make sure you have a good venting system for it to keep the heat down. All that light is worthless if the plants are just constantly heat stressed (I know that from experience)