When you craft shipping charge rules the carrier you will use does not have any impact on setting up the charge rules, if you wanted to indicate one shipping provider over another you could name the rule after the shipping provider to indicate this.
You say that you have three shipping rules for US customers based on a flat rate, what are the three different amounts and circumstances that you would like to setup these charges for.
Also, for the Canadian shipments how exactly would you like to charge the shipping and what factors exactly are you using for this?