It began in the present in a female psychologist's office with the main character waiting to speak to her.

The majority of the story is the background beginning possibly hundreds of years ago. Three children: two boys and a girl are possibly wizards. They age but do not die. They are part of many different events in life both good and evil. One boy and girl betray the other boy by becoming a king and queen I think.
The story ends with the remaining boy who is an elderly man meeting with the psychologist and I think her office burns and they escape together.

Have tried this on a couple of booksellers who review for BookBrowse and haven't been able to find a match yet, but hopefully someone will recognize it!