look at a sample website layout in the material using such a piece of code

< div class= "row" >
< div class= "col-sm-4 col-xs-12" > < / div >
< div class= "col-sm-4 col-xs-12" > < / div >
< div class= "col-sm-4 col-xs-12" > < / div >
< / div >

it seems unnecessary to have three col-xs-12 here?
as long as the page size is smaller than sm, each block will automatically wrap to fill up