Have you considered using a gallery block with grid design? If you use image blocks you are dealing with three individual content blocks on a row, which makes custom margin removal really hard. It would be much easier to use a gallery block and then look up the block ID & then give margin-left and margin-right values of -30px (or whatever it takes to remove the extra white space around the images). This however requires a business plan on Squarespace (because of Custom CSS) but I'd guess your site runs on business plan.
Or, if you wanna go with gallery block, you can simply apply our code & create full-width gallery in no time.