I have a few questions from chapter 3 that i need help with from the book fifth business1. Dunstan states, about himself, that "youth was not my...

Due to the number of characters allowed in answers, I will not be able to answer all of the questions.  Let's begin with the significance of names.

It's clear that being "twice-born", as Leisl calls it, has a great deal to do with having different names as a child and as an adult.  Dunstan and Percy-Boyd have different names as children -- Dunstan was Dunstable, and Percy-Boyd became Boy.  Dunstable was Dunstan's mother's maiden name (a common naming convention in Canada during the time of the novel), and therefore tied him firmly to her (it also is a compound word, and the two words "dun" - the color of a horse's coat, and "stable" -- a place where horses are kept, denote the less mechanized world of Canada before WWI, during which Dunstan was injured and had his dramatic vision).  After Dunstan becomes less enchanted with his mother the change to a new name seemed important.  The shortened name was suggested by Dunstan's fiance, Diana Marfleet, because "Dunstable" is hard to pronounce, she said, and the change brought Dunstan more fully into his adult self.

Percy-Boyd, by comparison, had an episode in his childhood which turned him off the "Percy" part of his name (some of the other children teased him about his mother's pet names), so he extracted "Boy" out of the name and kept it.  This is an example of how Boy Staunton didn't grow and change as an adult -- he remained a "Boy" his whole life.  Also, it refers to how Boy, and the changing 20th-century culture around him, grew to worship youthinordinately.  Boy was a mass of unrealized immature desires, covered up by worldly success with money and women.  He is in direct opposition to Dunstan's evolution away from his childhood -- Boy never leaves his childish personality behind.   

Your number 2) question refers to the investment advice Boy gives to Dunstan.  What Dunstan got from Boy was the equivalent of what we, today, would call inside information or "insider trading".  Boy, the head of a huge corporation and, during WWII, a government minister, was uniquely situated to give investment advice.  Given the fact that Dunstan was frugal, had no family, and lived at the school at which he taught, he was able to invest a large portion of his income.  This income enabled him, later, to support Mary Dempster in an insane asylum -- an impossibility for most prep-school teachers and writers with modest incomes from their books.  This made Dunstan able to assuage his guilt about Mary, and he even grudgingly acknowledged that he did owe Boy something for this advice.  That a portion of the money he made went to support the woman that Boy injured and made insane is an example of irony in this novel.  The money Dunstan made also made it possible for him to retire and live in Europe with Leisl and Magnus (in book 3 of the series, World of Wonders).

Hagiography is the word for the study of the lives and works of the Catholic saints.  Since Dunstan's primary scholarly work is in hagiography, it is not only his main interest, but partially his livelihood.  Dunstan, while remaining staunchly sceptical of organized religion after his childhood experiences with Canadian Calvinism, is fascinated by saints after his vision during the war.  He devotes a good portion of his life to investigating saints' lives, largely, it appears, because of the affect of Mary Dempster -- whom Dunstan believes to be a kind of saint -- has on his life.