There's not much to see yet, but the first Windows 10 "Redstone 3" PC test build is available to Windows Insiders on the Fast Ring.
As Microsoft officials said previously, the first few new builds of any new Windows 10 feature release include mostly plumbing changes -- some of which can result in build instability.
Today's new build, 16170 for PCs, is most about refinements to OneCore (the elements that are common to all the different variants of Windows 10), code refactoring and other under-the-covers work needed before Microsoft developers can begin checking in code.
Today's build may cause app and game crashing; error messages around updates being cancelled; and Narrator not working at all, according to the April 7 blog post outlining what's new. Those running Surface devices should especially be sure to check the notes before installing. The build also includes a number of fixes to some of the ongoing issues carried over from the Creators Update.
Redstone 3 is the next feature release of Windows 10 that will follow the Windows 10 Creators Update, aka Redstone 2. Redstone 3 is expected this Fall. Company watchers are expecting Redstone 3 to include Microsoft's MyPeople feature that was cut from the Creators Update, some improvements around Continuum, tweaks to the look and feel of the operating system and other new features.
Microsoft will begin rolling out the Creators Update to mainstream PC and tablet users next week on April 11. Windows 10 Mobile users who are slated to get the update will get the Creators Update starting April 25.