I pull over 20 oz from a 3x3 on most grows and got just over 30 oz on my last harvest (my largest in a 3x3 so far).
I usually do this with 4 plants, topped once above the 4th node to get 8 branches/colas per plant with an even canopy (screens help with this) and vegged for 8-10 weeks or even a little longer, the 30 oz harvest was vegged almost 12 weeks. I veg until the plants fill up the tent in veg and get about 3 feet tall before I flower them.
I’ve done this with many strains, most recently with ILGM Gorilla Glue - a very nice producer but any decent yielding strain can get you there.
As far as lights, I’ve pulled over 20 oz using a 600 watt HPS but I’ve moved to LED lights this year and pulling even bigger yields, the recent 30 oz harvest was with a HLG 550 Rspec and a HLG 30 watt UVA and a HLG 135 Rspec used for under-lighting, all together about 700 real watts of killer LED.
I use the Advanced Nutrients Connoisseur line with all supplements and the 3 bloom boosters they have in the lineup for different stages of flowering (Bud Ignitor, Big Bud, and Overdrive).
No CO2 enhancement, just lots of fresh air.
Here’s what a 30 oz grow looks like, they were about 4.5 feet tall (from top of soil) at harvest, light as high as it could go, maximizing the entire space and height of the 3x3x7 Gorilla tent.