>How many billions would a major bank loose in a lawsuit, if they did the sensible thing and refused to deal with 3% of the population?

Zero. Because FINCEN/KYC[0] laws in the US mandate identification for all customers.

Which means that at least 3% of the populations is "unbanked"[1]:

   Some reasons a person might not have a bank account 
   include:

   Lack of access via a nearby bank branch or mobile phone

   Minimum balance fees

   Distrust of the banking system, typically due to lack of transparency    
   regarding fees and deposit timing[1]

   No access to government-issued ID, which is required to open a bank account

   To avoid delinquent debts, such as creditors seizing the account in 
   judgements, or the government collecting back taxes or child support

[0] https://www.investopedia.com/terms/k/knowyourclient.asp

[1] https://en.wikipedia.org/wiki/Unbanked

Edit: Fixed subject/verb agreement (laws/mandate)/fixed quote formatting.