By default, the three column class switches to 2 columns on mobile, but we do need to update the rule to account for the new margin and we can add one for single column on phones
Add the rules below in the theme Global custom CSS rule box
.is-mobile .widget.content-3-col {width:48%;}
.is-phone .widget.content-3-col {width:98%;}
and if you want a single column on both tablet and phone only add the single rule below
.is-mobile .widget.content-3-col {width:98%;}