Hm, sorry to say, but you’re using elements and styling not the way they’re supposed to be used.
The thing that is happening in your case is that the footer is positioned at the place where it should be when the DIV and P with the classes
gr3 wouldn’t be positioned like they are now, but just at their static position. So 3 times 400px lower is the place where the footer is at right now.
If you want to move the footer up, you’ll have to find another way to create the 3 columns. And there are multiple ways to achieve that, in random order for example: table, inline boxes, floating boxes, flexbox.