Of course, I can imagine an illness that makes you sick for three days. How about food poisoning. That's hard to verify, and people generally don't like to ask too many questions about symptoms. An email to your boss telling him that you won't be working for the next three days might look something like this

Dear [name of boss],

I am writing to inform you that I will not be able to work for the next three days as I am suffering from severe food poisoning. I am currently feeling very unwell and cannot come to work. I will keep you informed of my situation as soon as I can assess whether it is improving.

I apologise for any inconvenience this may cause me and thank you for your understanding.

Yours sincerely, [Your name]
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