WYSIWIG with post grid

What I see on preview is what I want but not what I get when I use the post gris component to work with 3 columns on my laptop. It is ok with larger screen, or with wider screen (on mobile) but not with my laptop screen. (It is good on preview but not in the displayed published article).

I want to have a grid of 3 columns well aligned, but I get 2 columns on the first line, then 1 column in the right side for further lines, (see screenshots) but I am not able to change it and to understand why it happens and what I could do to prevent it to display like this (which is not good at all …)

The page I need help with: [log in to see the link]