1. Yes

2. You have two options

• use the API to integrate with the current login/reg flow with FusionAuth. This lets you keep your existing html pretty much untouched, you're just calling out to FusionAuth instead of the database.
• remove them and use the FusionAuth provided pages with OIDC. This lets you use the theming and localization capabilities of FusionAuth, including super simple social signon.

It's your choice based on what your needs are, either way will work.

I'd only recommend using SAML if you have an application which only supports SAML, not OIDC.

You'll also want to make sure that when someone registers with one of your applications in FusionAuth, they register with all three. I'd probably use a webhook to ensure that.

So my question is
1- can it be done with FusionAuth ?
2. if yes, in that case what about the existing login/reg flow already exist in 3 applications, do i have to get rid of them completely and than add fusionauth or let them sit as it is and fusionauth will provide another layer top of it?

And what will be best way connecting these applications using OIDC or SAML (i am not sure whats diffrence)?

Thanks

Do you mean using FusionAuth as the identity provider and connecting these applications using OIDC or SAML?

We've tested 100M users, but haven't pushed the boundaries of tenants or applications.

However, we have clients in production with 5,000+ tenants and 5,000+ applications.