I'd use corid first. All 3 chicks can drink it. I'd leave her with the flock. In the picture above, she doesn't look puffed up, but if there is runny, red poop cocidoisis is a possibility. If the poop has a strong, "sweet" odor it is also a possibility.
If she is fully feathered, and she...
Coccidiosis is a good possibility. Especially if she is "puffed up" and looks cold. A mama heating pad is a good, cheap alternative for a brooder plate. I'd urge you to make one, in order for this chick to be more comfortable, and to thrive.
Have you considered a brooder plate or a mama heating pad set up? That way she can get warm, without heating the entire area. I would give her a little vitamin water daily also. (Rooster Booster makes a great one, called Poultry Cell) Don't make it the only source of water, but give them access...