f you use this example for your main page, you should use a different Template for your Category archives, otherwise, WordPress will exclude all posts in Category 3, even when viewing that Category Archive! However, if you want to use the same template file, you can avoid this by using the is_home() tag to ensure posts from Category 3 will only be excluded from the main page:

Lorem Ipsum is simply text of the printing and type setting industry. Lorem Ipsum

Lorem Ipsum is simply text of the printing and type setting industry. Lorem Ipsum