The most common extracranial causes of canine seizures are:
hypoglycemia (low blood sugar level)
Since you said she has diabetes, is it possible that her sugar level dropped sharply those three times? You did say they were the same time of day.
Another cause - fleas and internal parasites (hookworm mostly).
Heartgard is one of the more tried and trusted heartworm preventatives. I had to use it in a slow-kill heartworm treatment of a senior rescue lab. I would never skip heartworm medications for either of my dogs after adopting a heartworm positive dog.
For flea control I use K9 Advantix - monthly during heavy tick season, less often in the fall. No reactions and no fleas or ticks.
My mother had a dachshund who had seizures, was on phenobarbital for years and one day she watched him pass a worm - not tapeworm but she didn't know what kind. He never had another seizure and never needed phenobarbital again.
If low blood sugar is the problem, keep some Karo syrup in the cabinet and rub it on their gums when a seizure starts or seems imminent. That’s what the veterinary oncologist had me do for my Lab. It works.