I am currently in about the fourth week of flowering and my plants are all between 4 and 5 feet, they are in 3 gallon pots and I am worried that the pots aren't big enough and the roots don't have enough room. Is it too late into flowering to put them into bigger pots or bags? Or is it worth it to re-pot and let flower a little longer?
I'd say it's probably too late to get any advantage out of it. To get any more root growth you're going to have to break up the root ball and this will be a nasty shock that will take the plants a week or so to recover from. I've had plants that large do fine in 3 gallons, the only issue is constant watering the second half of flowering. A dry out is disastrous and with a large canopy you'll be going from a full soak to a light bucket in less than 12 hours.
at this point it is to late to repot and would not do you any good as root growth is not the plants top priority any more is is putting its energy into the flowers
i always like to finish in 5 gallon pots bigger is even better cuz the roots grow so fast but space is always an issue