When i understand your question well you should read Less mixins and variables.
You will find the mixins to generate semantic grid columns in the less/mixins/grid.less
file of Bootstrap's source code.
To generate a div that overspans 3 columns you should use for instance:
div.three{
.make-xs-column(3);
}
Notice the mobile first nature of these mixins. With the above code the xs
mixin generate CSS code which will be applied for all screen width (no media queries).
When using:
div.three{
.make-xs-column(6);
.make-sm-column(3);
}
You div will overspan 6 columns for screen width below the 768 pixels (thexs
grid) and 3 columns above the 768 pixels, the sm
grid and up (explicit calling the larger grid mixins is not needed unless you want to overspan a difference number of columns). It's inportant to start the mixins for the smaller grids before the larger grids, see also: Bootstrap 3 mixin multiple make-*-column