Usually someone with medical training decides "Yes, we need to land now" or "It doesn't require diversion, but go in hot". If there is no one with medical training, then the pilots can communicate with doctors (Mayo Clinic?) to decide.

Hyperglycemia (high blood sugar) or hypoglycemia (low blood sugar) can both be serious conditions that require medical treatment, either insulin (for high) or glucose (for low).

The three hour delay would account for off-loading the pax and his luggage (I assume), reports, refueling (if necessary) and other unforeseen circumstances such as waiting for traffic at ORD etc...

The HIPAA Privacy Rule says you are not privy to medical reports and I am sure if you call the airline they will just tell you to pound sand. If something shall ever happen to you, which requires diversion, we can just hope others aren't pissy about it.