Sorry to disagree here, but much of what you've said is wrong, at least when it comes to discus.
First, beef heart is an excellent food for discus because of its high protein content. However, it does require more water changes.
This brings us to the water changes and nitrates. You are right that discus really require low nitrate levels. With three discus in a 45 gallon tank and feeding beef heart, I would say that at least a 50% daily water change is needed, if the discus are juvies and still growing (under 4"). If they are larger than 4", then larger water changes, less often, will suffice. Something like 75% every three days or so.
Now, for the bare bottom part. Many fish really don't care if there is a substrate or not. Discus are one of these types and could care less if there is a substrate. When keeping discus, it is very important to keep the water clean. One thing that can hurt the water quality is food and detritus getting stuck down in the substrate. With a discus tank, it is important to make sure this does not happen. Keeping a bare bottom tank makes it easy to siphon out the leftover food and detritus, which is why it is highly recommended to keep discus in a bare bottom tank. With growing juvies, it is pretty much a requirement.
Another point to touch on is the number. You say that you have three, but discus are schooling fish that do best in groups of six or more. Having said that, discus also need a tank that's allows about 10 gallons per discus, so a 60 gallon minimum.
Discus are not the easiest to keep, but they are fantastic fish if you can keep up with their maintenance.