If you want to keep your fish as best you can, you will reduce the number of species you keep and increase the size of groups.
Cory cats, for example, are grouping fish. Sure you can keep just one alive, but if you want them to thrive and see them at their best, you will keep a group. I normally recommend groups as 6 or more, but since this tank is so small and that would take up too much of your stocking space, at least aim for 3. Since all the other fish you want tend to stay to the top of the water, the corys are a great way to use the bottom. Cory's are also fairly low waste producers.
With the 3 cory cats you can also add 3 or 4 mollies and 3 or 4 guppies. You should have an understanding of how to cycle the tank though, and know not to add all of these fish at once. If you will be cycling the tank with your fish, just one of t hose species is plenty to start with.
Now, this is assuming you will be taking proper care of the tank with weekly water changes and vacuumingand knowing how to feed and more importantly, not overfeed.
The levels I have recommended are what I would go with without blinking, but many will tell you it's too much. They are not entirely wrong - if you feel you might not be able to keep up with maximum stocking loads, you should choose between the guppies and mollies and keep just one type - understocked tanks are easier to keep clean and healthy.
Having only 3 small fish in that tank is certainly understocking, but it's overkill and unnecessarily sparse.
God bless