I agree with Dragon and Sunlight. I have 4 emerald corys myself in a 10g planted tank and even with the four of them they are still VERY shy and get scared easily. One cory is not going to be happy alone, and in a 3g tank you are really pushing the bioload with just the 2 fish in there. If you are doing only a 40% change every 2 weeks your fish ARE eventually going to get sick. You definitely need at least two 50% water changes EVERY week.
If you don't want ghost shrimp, which in my experience die very easily, you could try Amano shrimp. I believe their life expectancy is only about a year though, and all shrimp are greatly affected by bad water quality and can die within hours.