Preface or first chapter.
Question: Wondering if my first chapter should be called the Preface. Chronologically speaking, it would be about the third chapter.
This first section starts out as an ordinary story, but, throughout, it hints at something exceptional that has already happened. It is intended to grab the interest of the reader for the purpose of reading the additional chapters. It ends with a shocking revelation that reveals the novel aspect of the entire book.
Subsequent chapters will show the hero's journey and tease out exactly what happened at the beginning of the story.
Would this first section be a preface or the first chapter?
Does that make any difference for page count of the completed novel?
Usually what grabs readers is an interesting main character - which is why the bias today is to begin the story with the main character rather than the overall story.
Prefaces usually begin the overall story - perhaps by showing the inciting incident - when that incident, though important, doesn't involve the main character. Many readers skip prefaces because they want to get straight to the main character, and for this reason prefaces are sometimes looked down on.
If your third event (chronologically) is more exciting, I would consider starting the story there and making this event Chapter One. But I would also suggest you use this event to introduce your main character.
If your first two chapters (chronologically) aren't that interesting, cut them. You can always fill the reader in on what happened later on, on the fly.
A shocking revelation, an interesting event, and an appealing main character sounds like a winning combination for a first chapter. If you can also create an authentic and intriguing narrative voice... what more could one ask for? (Okay, good grammar, spelling, punctuation, and a consistent point of view - but they go without saying.)