The only way I could see to do something like this without JS was to use an adaptive approach so that pixel measurements can be used to stretch the inputs. The problem with percent in flexbox or grid is that the child percentage cannot stretch the parent when the parents width is dependent on its child content (so unless you give the parent a percentage width then the child element cannot have a percentage width).
I’m 99% certain this will be of no use but I thought I’d document it anyway as some clever cloggs may see a solution that I missed 
At small screen it goes to one column but could be changed of course.