I\'ve got following html structure:
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What is determining the width of the middle div? As my rep score will atest I am not a very experienced developer, but I think this is the crux of your problem. If it litterally does not matter what the middle column's width is then a div ratio of 70%, 0%, 30% and 7%, 90%, 3%. are mathematically equivlent answers to your problem. So your solution could be as simple as:
Some elements here.
Some more elements here.
Even more elements here.