Ah the joys of resizing dashboard...we allow our dashboard re-size so have grappled with this problem quite a bit...
So the best way I've found to do this is to fix the width of some, or all, of the objects.
If you want the objects to be evenly distributed over the width of the dashboard...one way I've found it to set the fixed width of all 3 objects (in your example) to something very big, like 1000 px. What this does is that each object will remain equally distributed regardless of the size (up to 3000px - 3 objects times 1000px, when they no longer expand...if you have larger screens than this you can use a bigger number). I've done this on the "Equal Size" Dashboard
The other way I do it...is to only fix the width (or height) of certain objects, which allows the other objects to re-size, while the fixed ones remain fixed. I've done this on the "Some Fixed" dashboard, where the filters are fixed at 200px and the Avg Speed Slider is left to re-size (I've also added a blank container in which also can help keep the resizing element to look sensible on very wide screen, as that will also dynamically resize)
Hope this helps.
Thank you Simon! The first suggestion works perfectly.