Hi Susanne,
Small offtopic, you used very heavy images, for example, background image (feather-pattern.jpg) has 12MB (!) and should have max 900kb. Please consider optimization for all used images. This is KEY to get good score in Google, and mobile users see all elements fast. Otherwise SEO speed disaster.
About your main question. If you created 3 columns in row (section) now you have to set their width in each view: Tablet, Mobile. Here is just example for header, but look and read "green box"
for example, if you want to keep 3 columns in tablet , you have to use col-md-4 for all 3 columns top1, top2, user1.
4+4+4 = 12, right.