How then? Taran asked. Could The Book of Three deceive you?
No, it could not. Dallben said. The book is thus called because it tells all three parts of our lives: the past, the present, and the future. But it could as well be called a book of 'if.' If you had failed at your tasks; if you had followed an evil path; if you had been slain; if you had not chosen as you did — a thousand 'ifs,' my boy, and many times a thousand. The Book of Three can say no more than 'if' until at the end, of all things that might have been, one alone becomes what really is. For the deeds of a man, not the words of a prophecy, are what shape his destiny.

Chapter 21 - The Chronicles of Prydain (1964–1968) - Book V : The High King (1968)