In my small 3 gallon tank I had 3 Zebra danios and 1 guppy. The water is cycled and a filter is running.

One danio got sick and died. It was not able to hold balance because its tail would go up and head go down and it seemed like it struggled to keep it up. It also would not eat as much as before and move less.
Now after a week this is happening to another danio.
The guppy seems to be doing fine.

There are no visible ick spots or any other visible signs on the fish.
Only one possibility is that the guppy and this fish might look a bit more pale on sides of the body.

I've no idea what it might be so I dont know what medicine I could buy to help them.

The danio is a fast moving, active fish. It might feel pretty cramped in a 3 gal. Try adding aquarium salt and/or just moving to something larger..like at least 10gal, assuming its empyty so you're not gonna give the sickness to other fish if there is one.. =P

Those symptoms are kinda general.