I have tank with the exact same dimensions as yours and I just changed my substrate to fluorite. It says each 7kg bag of fluorite is sufficient for 10 gallon tank. I think our tanks are considered tall tanks so I'm not exactly sure you need 3 bags. I bought two. I could use one more but 2 seems enough for now anyways. If you want deep rooted plants, I think 3 bags are perfect. I currently have Amazon sword and Crypt. Wendtii in substrate for now. I'm getting more. One thing you have to remember about fluorite is that you have to wash it well enough so that the clouding in your tank during the initial set up is minimal. I have black sand fluorite and red one I think. Red one was much bigger chunks so it took less time but the sand one took longer. There is no end and I stopped when it was half decent clear when washing them and when I put them in the tank it still clouded up pitch black. Some people claim it takes several days to be able to actually see anything in the water but it took me an hour for the water to be white cloudy and able to see everything. In about 3 days the water was crystal clear. A lot of people say that when they rearrange their plants the water becomes black again but I just rearranged some plants and the water didn't get cloudy. When you have fluorite, it's a very natural look and it definitely makes the cardinal tetras and other colorful fish stand out. The other good thing about fluorite is that you don't have to layer them and add extra sand or gravel on top.
Edit: Actually mine is 30 by 12 so I guess you can use 3 bags. I can get away with two because I'm going to put all the deep rooted plants in the back and I made a slope going up towards the back.
