IMO/E the 3g is just fine for a single Betta....
I would use enough of the IAL to stain the water to a nice amber color, tear them up in pieces if you don't already and if you have carbon in the filter remove it and IMO I would turn the filter off the current can cause fin damage and IMO/E more water changes are better anyway especially for the CT delicate fins.
You can pre-mix your water and keep IAL in the pre-mix, you can even make a tea out of the IAL just pour hot not boiling water over them and let them steep 5 min or so, dump that water and repeat to get all the tannins out of the leaf and keep in a clean jug, dilute and use for water changes.
I use native oak leave, they do the same as the IAL, I have used the IAL in the past and I used 1 large to 5g of water and left them in the tank until they fell apart, I do the same with the oak leave, I like the nylon idea that would make for easy cleanup...lol....