3-6-3 Rule

What Does 3-6-3 Rule Mean?
Slang used to refer to an "unofficial rule" under which the banking industry once operated, which alludes to it being noncompetitive and simplistic.

The 3-6-3 rule describes how bankers would give 3% interest on depositors' accounts, lend the depositors money at 6% interest and then be playing golf at 3pm. This alludes to how a bank's only form of business is lending out money at a higher rate than what it is paying out to its depositors. 

Investopedia explains 3-6-3 Rule
Many attribute the problems faced by the banking industry during the events that lead up to the Great Depression as reasons why the government implemented tighter banking regulations. These regulations controlled the rates at which banks can lend and borrow money. Unfortunately, the regulations made it difficult for banks to compete with each other and the banking industry became stagnant.

However, with the loosening of banking regulations and the widespread adoption of information technology such as the internet, banks now operate in a much more competitive and complex manner. For example, banks are now providing insurance, brokerage and other forms of financial services.