Are you talking about a new 3g tank, or the same one your male is in?
If it is a new one, a female would do perfectly fine alone in it; I do not suggest any other fish, amphibians, or invertebrates, as that is a very small tank and ammonia will build quickly in it with more bioload than just one betta. Even though oxygen needs allow bettas to be kept in small tanks, water quality still mandates that, unless you plan on doing very frequent cleanings, bettas should probably have a good 2.5g to themselves. Since any reading I've done on snails and ADFs suggests they should also have a good 1-2.5g each as well, I would strongly caution you against adding any tankmates. Perhaps you could just plant and decorate the tank well, giving your fish a nice place to live and making his/her tank more interesting without stressing any of the inhabitants. (Note that opinions will vary on this, and many people do not mind overstocking a small tank a bit by adding snails or ADFs. I just feel that water quality is much more important than a more exciting tank)
If this is the same tank your male is in, I'm sorry, but you can not add and female. They will fight to the death. Male bettas are 100% solitary and should never be kept with other males or with females. Females usually prefer to live alone, but are capable of living in groups of 4-6 in a minimum of 10 very heavily planted gallons with plenty of hiding spaces.
Absolutely no other fish could go in a 3 gallon tank, either way. A three gallon tank is sufficient for no species but the betta, as all other fish grow too large, or require group living to remain happy and healthy. If you wish to have a community tank, you'll need to shop for something larger that you can filter, heat, and cycle properly. I personally do not suggest this until you are more researched and know more about proper fishkeeping.