It helps if you stop looking at it like "What's the minimum amount of water I need to give them at one time?" As mysticepipedon said, what's important is to make sure you are saturating the ENTIRE pot.
One thing you could do especially since you're in fabric pots, is take something like a 5 gallon bucket or something big enough to fit those 3 gallon fabric pots in, and fill it with water, take the fabric pots and dunk them in the water. Let them sit in there for a minute or two and really soak up as much water as they can.
Or, you could put drip trays under the fabric pots and keep watering until it will literally not hold another drop of water.
The excess water that doesn't get soaked up by your fabric pot/soil, is known as "Runoff". The general rule to watering in pots is to water until you see at least 10 to 15% of what you put IN to the pot, run out the bottom. That means it's saturated and won't absorb any more.
Next, you need to SAVE that "runoff" and test it's pH. This will give you a better idea of what the pH is doing in your soil, and my bet it's off in one way or another, and a big part of what's causing your root issues.
So start there and we'll talk about additives that might help your roots after we get some results on that runoff pH
