MULTIPLE ACCOUNTS SAME EMAIL

I have a customer that has 3 accounts (each account has 2 or less terminals) and wants to use the same email for all 3 but when I try it says the email already exists.  I found a work around but it is not ideal.

Can we allow them to access all 3 accounts with a single email with a drop down option like our enterprise solution without the enterprise level options

Maybe offer an Enterprise solution for Core Subscriptions