In General, faced with such a problem when coding a responsive site. The width of the website floats in the range from 320 to 1600px. There are 3 blocks. It is necessary that these blocks took the entire allotted area and thus have the same width(auto) and spacing(30px). When you reduce the window size the blocks should be compressed in width, and margins remain unchanged.
Implementation without a flexbox'.
Need Your help, dear friends! I would be very grateful if you help!