You'll need to cycle the aquarium. Keep adding the same amount of food daily. Test ammonia and nitrites until the ammonia reads zero and you are getting readings of nitrites. Then start testing for nitrates. The process is complete when you get readings of zero on the nitrites and have some nitrates. It can take a few weeks.
For a 3gal, I would recommend a betta, or a couple male guppies, or male killies like Santaa suggested. You could also keep shrimp. 3gal kind of limits your choices. I would suggest a larger tank for a first time, it allows for more error. I would get a small heater. Don't worry about pH, or hardness/softness of water. Most fish will adjust to whatever you have.
When you do your water changes, you don't want to rinse out your gravel. That will kill all of your good bacteria and you will have to re-cycle your tank. I don't use undergravel filters, so someone else that does will hopefully come along and tell you how to do the weekly maintenance on them.