Instagram widget - 3x across

I’m keen to utilise the readymag widget to embed specific Instagram posts and fit 3 across on desktop. Currently, I’m finding that they’re scalable up, but not down so I’m not able to apply them to my page as I’d like.
I know there are third party providers out there that handle this sort of thing, but is there some code I can apply that will change the container sizes and allow me to squeeze 3x across? Soz if this is obvious, but coding is not my strong point.
Visuals attached are (1) what I currently have; and (2) what I’d like to achieve…