Does Thinktecture IdentityServer V3 has support for SAML or does it only support OAuth?
IdentityServer v3 supports any Katana-compatible Owin middleware for authentication. With a compatible middleware for SAML2 such as Kentor.AuthServices IdentityServer 3 can authenticate against external SAML2 Identity Providers with Idsrv acting as a SAML2 Service Provider. I wrote a blog post on how to do it.
There's also a plugin interface which would allow IdentityServer v3 to act as a SAML2 Identity Provider but as far as I know there's no implementation available.