For a 3x3 tent you want at least 250 real watts and more is better if you want to maximize yields and bud density. I find the sweet spot at around 400-450 watts. That said I have about 800 watts on tap in my 3x3 flowering tent but rarely use more than 500-600 watts.
My flowering tent light setup - a HLG 550 rspec and and a HLG 260 rspec with a couple HLG UVA 30s.
Just the HLG 550 would be more than enough for most growers in a 3x3, and even the the HLG 260 by itself would do alright.