It is because your stlying has borders/padding/margins that apply to the cells.

This is a common question across all the column grid modules. You can find a whole number of posts on this forum about how to solve the issue. You need to remove the borders/padding/margins from the cell. that will allow three cells to fit on a row.

Then if you are using the most recent version of snaf you can apply the styling to the inner box that is added in to the code.

As you are using a TM template I am not that keen to wade into the code more than that :-)