On the Size panel are four sizing bars, one for each breakpoint range. The bars are measured in 12ths (in other words, each bar has 12 segments).
You can see the Placeholder is set to be 4/12s wide for the three widest breakpoint ranges.
Each of the three Placeholders you've added has this same setting - they are each taking up 4/12ths of the available width, so altogether they are 12/12ths wide.
We can adjust the settings on the Size panel to customize the layout on this tab so it has four columns instead of just three.