Hard to know what's causing their decline. Did you have an existing flock that you introduced these to and are all of those healthy and thriving?
Testing is the best way to find out what's happening, but if you want to try an antibiotic, then something like Amoxicillin would be your best bet...
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Are the hens new to you? How long have you had them?
What is the timeline of losing the 3, are you seeing them all within a week or so of one another or has this been going on for a good while?
Corid won't hurt them, but it's not an antibiotic, it's a Coccidiostat to treat...