For just 3 it's a very good amount of space, and you have some "stuff" in there but yes you lack vertical breaks in line of sight - a bird eating in one corner can see clear to the other corner. Maybe lean a few pallets against a wall here or there, or like you mentioned, build something around the box in center since it's already there, and breaking line of sight in the middle covers a lot more space than just having something small on one side or another.9x12. 3 birds total. The primary feeder normally goes where the red dot is, so we added a secondary one on the other side of the dig box near the waterer (we move the primary to Ginger's crate while they're separated).
One thought I had was adding another 6" tier to the dig box, maybe even with a 2" shelf topper. That might serve as a visual barrier between the feeders. I could also attempt to add a center wall where one of the roof supports is.