Show only 3 entries/row in Summary Block on desktop?

I have two issues I would like to see if anyone knows how to solve, any help is much appreciated!

First, I am wanting the summary block on my site to show no more than 3 entries per row. It does this when shown on a smaller screen, but once I view it on a larger desktop it extends to 4 entries per row. Is there any way to specify this?

Second, I would like to decrease the side margins on the site when viewed on a desktop to prevent it from looking as stretched as it does now. Is there a way to just adjust the body margin rather than side margins? I basically just want the sides to increase as the site extends, with the content still centred to 3 entries per row in the summary block.

https://ukulele-vibraphone-9ele.squarespace.com/
password: blackwatch

Thanks!