Luke Smith tweeted this out yesterday (haven't seen a billion posts about it already). Looks like Raid Challenges are confirmed along with how we will find the final 3 fragments.
"Raid Challenges are not currently active. Final 3 Fragments are among their rewards. More info in several weeks"
https://twitter.com/thislukesmith/status/659138544133259264
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Lol weeks. Is this insurance for FO4?