Howdy Ruger15 and Welcome to BYC, sadly the circumstances of you joining are not good and I am sorry to read of your loss
I am definitely not an expert on what ails chickens but hopefully my responding will bump your unanswered question.
Sadly, I probably have more questions than answers but your answers may help someone else to assist you.
My first sign that a chicken is unwell is usually their poop. Have you noticed a change in their poop at all? The following is a good guide to normal and abnormal chicken poops:
If they look abnormal, a picture of them might be helpful in diagnosis.
Have you wormed them?
Do you use preventatives for mites/lice? Have you inspected them for those nasties?
Are their combs a nice red or a pale pink?
Do you free range? Any chance they are getting into and eating something they should not.
No access to anything mouldy?
Do you know if they were vaccinated against Marek’s?
Were they only young when you purchased them which is why they did not start laying until August?
I have heard of chickens suffering from sudden death and have actually had one myself with absolutely no symptoms the night before and gone the next morning which I can only put down to sudden death or maybe something she ate or something bit her; but three in three weeks sounds like something else.