Adjusting padding in 3-column layout results in pushing third column to next row

I am simply trying to add background colors to three columns and add margins/padding to the content within each column so the words are not flush with the edge of each column. I’d like white space between columns and the effect of three boxes across the width of my page. Whenever I adjust the padding/margins within the columns, the third column pushes below the other two instead of all columns scaling responsively and staying as a row of three columns.

On another note, adding a background color or border to any textbox does not work,and I am using your theme completely out of the box with only the plugins you include.