Widht of home page not working

I use a home page with a grid blogpost and 3 columns. On my normal computer, this works just fine. But when I use a computer with a larger screen, the two first posts get pushed to the left, while there is just a blank space for the third post, which then goes below the two others. How do I change this? It makes no sense that the site shows as it has less space when it is shown on a wider screen.