How to change some code

Hi, I have just started working with wordpress and am tweaking the homepage. I have encountered the code:

(‘type=post by post & limit=7’). Basically, it lists the 7 most recent of my posts on my home page. I want to exclude 3 different category id’s from this listing process. Can somebody be so kind as to explain to me how I can exclude these categories. With thanks for your kind help, Ken.