There are two borders that move: the one joining the right edge of the first panel to the left edge of the second panel; and the one joining the right edge of the second panel to the left edge of the third panel. (I'm assuming the leftmost and rightmost borders will remain fixed at the edge of the browser window).
The horizontal positions of these two borders are going to be expressed as a percentage of the browser window's width as measured from the leftmost edge of the browser window. Thus, if we say that the first border is positioned at
50%, this means that it's exactly in the centre of the page; and if we say that the second border is positioned at
80%, then there's four times as much space to its left as there is to its right, i.e. it's 80% of the way across the page, with a 20% margin on the right.
At these positions, the three panels would occupy 50%, 30%, and 20% of the page width respectively.
Bearing all that in mind, all you really need to decide is how much space you want each panel to take up, and translate that into two numbers that represent the horizontal percentage-positions of the two borders.
65 as the values that make up the variable
middleDiv=divs.filter(x=>(x.style.left!="0%" && x.style.right!="0%"));