To define a 3 column grid you could use the customizer or download the source set your less variables and recompile.
To learn more about the grid and the columns / gutter widths, please also read:
In you case with a container of 960px consider the medium grid (see also: http://getbootstrap.com/css/#grid). This grid will have a max container width of 970px.
When setting @grid-columns:3;
and setting @grid-gutter-width:15px;
in variables.less you will get:
15px | 1st column (298.33) | 15px | 2nd column (298.33) |15px | 3th column (298.33) | 15px