As per an announcement on the White House blog yesterday (emphasis mine):

Consistent with the commitment to the Open Government Partnership, later this year the United States plans to publish a third Open Government National Action Plan (NAP) including new and expanded open government initiatives to pursue in the next two years. The first U.S. NAP was published in 2011 and the second NAP — which is still being implemented through the end of 2015 — was published in 2013.

Members of the public are invited to contribute and can do so via email, on Twitter or via Hackpad, which includes a separate document on Public Participation + Open Policymaking.