Can ADFS 3.0 delegate authentication to a non-ADFS identity provider through a federation trust relationship?
Yes. We see customers regularly interop with multiple identity providers. These span across traditional federation vendors as we as cloud IDP's as well as cloud SaaS apps.
Check out http://social.technet.microsoft.com/wiki/contents/articles/2735.ad-fs-content-map.aspx#Interoperability_with_Non-Microsoft_Products. Some of the content is old, but the concepts are pretty much the same.
Thanks // Sam (@MrADFS)