Before you get started on reading this issue published by Marvel Comics, you should read the second issue of Powers of X. This means that Powers of X vol 3: This is What You Do is the fifth issue you should read, to read Powers of X and House of X in the correct order.
Synopsis of Powers of X vol 3: This is What You Do
The X-Men interrupt the worship of the machines at the Church of Ascendancy to attack.
Omega Sentinel attempts to persuade Nimrod the Lesser to investigate the attack, but he’s dismissive. As a result, Omega Sentinel goes to confront all by herself.
The result of the conflict was disastrous: both the X-Men and the Sentinels ended up dead.
Apocalypse, War and Famine step into the mission of getting their hands on an information crystal. They find it, but they are immediately caught by Nimrod. To escape, Famine opens a portal, while Apocalypse decides to stay behind to keep Nimrod busy for as long as possible. As a result, War and Famine escape, while Apocalypse falls to Nimrod.
Following Apocalypse’s wishes and consented by Moira, War kills her, so she can now start again, but this time in the possession of when Nimrod steps into scene.
What to read after Powers of X vol 3: This is What You Do?
Once you’ve finished reading Powers of X vol 3, the next one you should read is House of X vol 3: Once More unto the Breach.