You have many issue and most of them are very minor bugs.
There are too many nested div which are only making it more messy always keep it simple.
Too many media queries . You don’t even need media queries for this.Since there is a rule to use one media queries you should just use one.
The navbar issue was that you have to make it sticky so that when you scroll it stays in the top.
The Last 3 columns problem was that you closed the flex div right after
h2 which was causing the issue that’s why you should not use too many divs.
Since there are many issues i forked your project and fixed all the issues and you can take a look at Here