Discus and neons really aren't good tank mates. Discus like 82F+ degrees, anything less could very well lead to disease and other problems. Neons don't do too well in temperatures over 79F.
Discus can be kept in 40g tanks. Generally, 10gs per fish is okay (in barebottom), but in planted tanks it would be a lot less. Three would be pushing it. Personally, I would buy an adult pair, they don't have to be proven, just bonded.
Another option would be to go with angelfish, they may be okay with neons, but have been known to eat them.