Looking to make these 3 image blacks fit the full scrren

Dragging to the edge of the screen will mean that the outside images take up 1/3 of the max section width, plus the margin, which varies based on the browser width. If you want this full width you have a few options, you could use an auto-layout list section with a slightly different design:
https://support.squarespace.com/hc/en-us/articles/360057763852-Auto-layouts
Or you could make the section grid full width with a piece of code:
https://www.launchthedamnthing.com/blog/6-tips-for-fluid-engine

Try this Custom CSS:

section[data-section-id="634d6b32eed6b431d897a435"] {
  .fluid-engine {
    --sqs-site-max-width:100vw;
    --sqs-site-gutter:0vw;
  }
}

Let me know if that helped!