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?

Most do.

For example: https://www.reddit.com/r/Banking/comments/1csl00q/any_banks_...

4.2% of US _households_ are unbanked. Banks refuse customers for many less extreme reasons than “can’t verify identity” which is in fact a requirement for them with all of their federal and state regulators.

>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.

Further reference for the ID requirements:

https://www.bankhelp.gov/help-topics/bank-accounts/required-...

> About 3% of the us adult population doesn’t have government issued photo id.

Add on top of that undocumented people or the issues surrounding the Native American population and their partial autonomy rights, and it becomes a mess very very quickly because it won't stay at "about three percent".