It is a good practice to analyze your page in at least 4 sizes, Extra Small(Xs), Small(Sm), Medium(Md) and Wide(lg), so if you want to use only one class for example, col-Md-4 means that your element (div) will have a width of 4 columns starting (mobile first) medium-sized.
This means that in all other smaller sizes, the width of 12 columns will be set as there is a standard size that is 12.
If you do not want the size of 12 columns to be applied in a certain screen size it is recommended to include the size you find convenient.
Current browsers have tools to help analyze your page in various sizes. In Chrome and Firefox just press F12 to open a number of useful tools for this process.
Interesting is the way you organize it. To be honest, I had never seen it like this. I always used / saw only one class at a time. Has some advantage over the use of the three rather than just one, since they are (theoretically) equivalent to each other?
– Hamurabi Araujo