Random question I just thought up.

So say you have a plant that is a month to six weeks into flower, pretty well into it. You have it in a three gallon pot, and you think to yourself that maybe five gallons would have been better. At this late in the game, if you transplanted it, maybe added some supersoil in the bottom, what would happen, if anything? Obviously it would have been better to do all this in veg, but could this help yields any?

Also, totally unrelated, with watering, what is the best method? I usually give a plant like 1/4-1/2 gallon every day depending on size, but I've heard ppl also dry em out a bit then just drench them once every few days. Seems more natural like that, but is there really any disadvantage to just watering every day?

Do not transplant this late in flower.

Watering everyday does not promote root growth. You should water thoroughly then let your plant dry out. You want the roots to seek out water in your entire pot.

Ex: If you were going to water everyday, why not just use a quart bucket and save yourself $ on soil ? See what I'm getting at?

You COULD transplant it, but you probably wouldnt get too much more of a yield.
I read that people keep watering till they see it drain out the bottom. I dont like doing this because I feed 2x's a week. I give my 5-7 g pots 1 gallon a day. I can come back in an hour and see small puddles on the floor. I have plants outside and in veg room with high winds outside and high fans inside. I never drench, I dont need to I check on them every other hour. And I also give them a nutrient flush on week 4,7,10 and 12

Watering every day and not letting the soil dry out in top of the pot, can also attract insects to your soil, I water every 2 day and soak the soil a little so it can hold the moist about 2 days

I have re-potted during mid-flower with success, but I had a reason that made it worth the risk. Doesn't sound to me like you have a reason. A 3-gallon is plenty big. Upping now won't help your yield. If you're worried about soil depletion, read up on nutrients. Nutrients and lighting are the best way to increase your yield (and starting with a high-yield strain to begin with).

I water about once every 5 days or so once my girls have reached flower and are in their final pots. I let the soil get dry enough that it starts to pull away from the sides of the container, when the leaves are visibly just starting to look thirsty. Then I water by pouring a little (maybe 2 cups worth) just to get the soil wet, followed by a drench (but not a flush!). By the time I make my way around the room with the pre-water (the 2 cups worth) and get back to plant #1, the plants are ready for the drench. The pre-water with the small amount helps to create a water path so that all of the water doesn't just run down the sides and out the bottom. The idea is to get all of the soil wet rather than let the water find a path through and out the bottom of the container, which it will do when you let the soil get as dry as I let it get. Make sense?

Your plants NEED to get thirsty and search for water. If you water every day or too soon otherwise, they won't get thirsty and the roots will be weak. You also risk root-rot and a bunch of other nasties that result from over-watering. Some people over-water by watering too often with small amounts of water, and the water never or seldom actually finds its way to the roots, resulting in an over-watered plant that dies of thirst.

I use 3 gallon air pots and water once a week. just because the top soil looks dry does not mean a bottom is dry. watering every day will promote root rot your roots need air to breathe and grow. Live and learn dildo.