Sometimes they get sick with even the best care. I also agree with you on less water changes but clean water is more important. But in a three gallon, 2 weekly changes are recommended so I'd either up water changes or be 100% sure the tank (if it has a filter) and the live plants are keeping the ammonia and nitrites at 0 between water changes.
There's a lot of factors and I'd need exact details to tell you the things I believe MAY have been wrong, but a few of them are temperature (is his temp 78-80 AND stable?), food type (Omega One and NLS are good brands), and possibly age and genetics. I had a girl pass away from a temp fluctuation caused by a 1 hour power outage and came to the conclusion that her genetics must not have been as strong because none of my other 4 fish fell ill.
I hope this helps?