Keeping anything at a constant level is difficult in a 3 gallon. Make sure to keep up with your water changes!!! Get a small submersible heater and like Lee said, the damsel isn't the best fish for that size aquarium. Neon Gobies and other "nano" gobies should do fine for you. If you keep a close eye on your water parameters and like I said, don't slack on the water changes.
Best of luck, have fun, and welcome to MAAST!