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!

Try this thread:

Not sure I understand how you want to adjust the margins on desktop, you can change the page width across the website to wider, or use CSS to make certain sections full width.

Thanks,  that worked in terms on only showing 3 columns but the content is now a bit scattered, any idea why? I attached a screen shot, it should be 2 rows with 3 columns in each. Thanks!

Regarding the margins: When I view the site on a desktop the site looks too wide, with the line lengths in the text blocks longer than I specified. I would basically want the layout when viewing on smaller screens to carry through to desktop, with the sides increasing rather than the content being spread out to fit the screen. Hope that makes sense! I was thinking of using a break point, but unsure if that would work or which code to use for it. Let me know if it's still unclear, I've just encountered this issue so not sure how to word it!

It looks fine to me. Did you solve it?