I’m assuming this is the course you’re now following, in which case my advice would be to go more slowly and not try to add everything at once.
Set up your three columns. Add the first item to your main column. Does it look right? If not, why not? Sort that out, and when you’re happy with it, add the next item and repeat the process. At every step, stop and validate your code:
HTML - https://validator.w3.org
CSS - https://jigsaw.w3.org/css-validator/
Don’t add more content until what you already have is right.
Once you have the middle column done, you can turn your attention to one of the side columns and repeat the process. Before you do that, however, you need to decide what you want to happen with your content at screen resolutions too narrow to comfortably display three columns side-by-side.
Planning should be the first step for your site.