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I am not sure which 3 bars you're referring to, but if you mean to open the navigation menu (often refereed to as a hamburger icon) the full-sized navigation menu will automatically convert to this on smaller screens where it wouldn't fit.
There isn't a separate navigation tool, rather the navigation menu is determined by the Pages menu and individual Page Options. Make sure you have the appropriate pages set to show in navigation, if you do and it's not working contact support directly so they can look into the issue. If I am way off it would be helpful if you share the URL to illustrate what is happening.