The AAD P1 or P2 license is required while trying to register an app is AAD and using the SSO feature for that app. In case of P1 and P2 license, you would be able to add unlimited SAML based apps in your tenant and utilize the SSO feature available for them.
Regular E3 licenses wont help here. Its better to have either P1 or P2 license in your tenant.
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