Three deaths in four days means I would want you watching for coccidiosis rather than only fixing the brooder: lethargy, hunched and fluffed, watery or bloody droppings, and sudden deaths. It is common in young chicks and it tracks damp dirty bedding.
Isolate anything that looks unwell, get the bedding dry and clean, keep notes on ages and symptoms and timing, and ring a vet or your extension service if the deaths continue. Some poultry diseases are reportable and the notes are what make that conversation useful.