Welcome to the forum Kris.
Distilled water has no minerals at all in it. If you have exceptionally hard water, you can soften it a bit by mixing distilled water with your tap water, but straight distilled water will kill your fish. Fish need a reasonable mineral content in their water to survive. Even "spring water" is often too pure for fish to live in. It depends on how the water is filtered and treated before it comes to you. I would at least test so called spring water before using it in a tank. When I worked doing some water testing in an industrial lab, I took a few samples of the bottled drinking water that was in the water cooler. It was almost as pure as distilled water. I am sure each supplier has their own way of making drinking water but that stuff would have been way too low in TDS for most fish to survive. If your tap water is not too bad, use it and the fish will be fine.
In a 3 gallon tank I would be changing a gallon a week, especially if you have a significant fish load in it. Anything more than a single betta is starting to get to be a heavy biological load for a tank that size.
