Agreed, keep the clutter out in the run away from walls or fencingThose are pretty big chicks for 3.5 weeks, are you sure on the age? My answer would've been "brick sized" and that's 2x4. I have chicks 7, 8 weeks of age still managing to barely squeeze through openings that size.
Do you have ample clutter in the run that doesn't have pinch points (like I don't have any clutter up against walls, so chicks can run all the way around) as well as feeders spread throughout the area? That's how to prevent chicks from being cornered and reduce guarding of food.