Please do not use 'monitors' in phrasing - always refer to them as either Primary and Secondary PCs.
First, increase the size of your grid:
Next, I don't know where you are trying to drag any third PC to - where on the grid are you?
If that fails, and after you have expanded the grid, send me a video of the issue. Your PC names are not a security concern if you have any. You can change them if you do not agree.