What is the purpose of the first three articles of the constitution?

To set up the government as a Republic with 3 separate branches.  

The purpose of the first three articles of the constitution is to help establish the three branches of government the legislative, executive, and the judicial branch. 

The case was a u.s. supreme court decision that defined the scope of the u.s. congress's legislative power and how it relates to the powers of american state legislatures. the dispute in mcculloch involved the legality of the national bank and a tax that the state of maryland imposed on it.