I have started writing my autobiography. I am going to describe 3 very important life events.
Now, should I write about all three life events in one book or should I write them separately?
If you want to describe one important event in one book, then that's not an autobiography; it's a memoir. There's absolutely nothing wrong with that, and you can certainly include information about your life aside from that event in the book.
An autobiography would be the entire story of your life to date, and you would generally go into less detail on each event, focusing less on the event itself and more on the way that event affected your life.
If you have three significant events to write about, I think you could justify writing four books - three memoirs, one about each event, and an autobiography.
An autobiography will generally tell your whole life up till now, you wouldn't write a whole book for a single event.
Furthermore, you'll want to include more than just three events if you're going to draw people into the story of your life and see how you develop as a person.
Is there a compelling reason why people would want to read your autobiography? Usually people who write autobiographies do so because they are have had a highly eventful life, something out of the ordinary.
It's worth considering what will make your autobiography compelling to read and ensuring that is a clear theme throughout. Also, consider the development of the story, how your life and personality changes over time.
This is perhaps wandering from the point of your question but is hopefully helpful.