What up @Jackson? A p300 should light out a 3' x 3' x 5' gorilla tent rather nicely. The core coverage area is 3' x 2' so you will have 6" on either side that don't fall under that area. Our p450 has a core coverage area of 3' x 2.5' which will give you slightly better coverage and more intensity. You can always get a p300 now, and pimp that tent out with another p150 or p300 down the road.
It's not really the pot size to consider but how big the plants get.I have had some plants over two feet wide so even if you place them close to each other they would cover a five or six foot wide area.If your going with smaller plants you could possibly do three 5gal pots and then rotate them every few days so they all get even light access.Check this link out and it will show the p300 foot print.https://platinumgrowlights.com/products/p300
Jack, just try...do you plan to LST the plants?
With my P 600 i can cover a 1.8 meter x 1.4 meter on the right side of my groom easily with 6 big plants and getting great results, but it's tight there...