How do you get all the beginners in diomand without trading

If, by trading you mean with other people, then you can get multiple copies of the game and jus trade with yourself using common Pokemon. But that's a bit expensive. Your best bet is to buy a gameshark. I believe there's a cheat that it provides (if you download content from online, that is) that allows you to catch literaly any Pokemon there has ever been in the wild by tossing out a number of masterballs (there's another cheat on it that gives you an infinite number of any item also) equal to the Pokemon's national dex number. For the starters in Diamond, these numbers would be 387, 390, and 393. Getting the starters using just the game by itself? Can't be done.