When more than one chicken becomes sick, and multiple deaths occur relatively soon afterward, it narrows down the possibilities.
One of the most common causes of sudden illness affecting multiple individuals in a flock is cocciiosis, and if you are in the northern hemisphere, springtime is a period when it often strikes.
That would be the first thing I suggest treating for. Depending on where you live in the world, it will be either easy or a challenge finding the proper medication.
The second most common possibility is heavy worm load, also a spring time risk. I suggest treating for worms at the same time you treat for coccidiosis. It requires a worming medicine. Again, where you live will determine how easy it will be to find this med.