If you don't want to use LDAP or EEM authentication. you can create local users in users.xml like customerA, CustomerB, and CustomerC.
<users>
<user password="" name="SaasAdmin"/>
<user password="" name="Admin"/>
<user password="2a.1000.a9hZlUjIZUVV4vMjkv3BtA==.Ysxf7wwkpbWLN6reklfb9jaXoQ==" name="Guest"/>
<user password="2a.100000.uJLqdhmvSNCNiaCjxPDadeA==.m7lQm6pkgr0KDFM3bpuTWY3A==" name="CustomerA"/>
<user password="2a.100000.uJLqdhmvSNCNiaCjxPDadeA==.m7lQm6pkgr0KDFM3bpuTWY3A==" name="CustomerB"/>
<user password="2a.100000.uJLqdhmvSNCNiaCjxPDadeA==.m7lQm6pkgr0KDFM3bpuTWY3A==" name="CustomerC"/>
</users>
after that create domain with the names like Application A for Customer A same as for B and C
<domain name="Application A" description="Application A">
<agent mapping="ApplicationAHostName\|(.*)\|(.*)"/>
<grant user="CustomerA" permission="full"/>
</domain>
Create same for B and C also.