From Josh Olin, community manager, in the Treyarch official forum...As with the previous two Map Packs, Map Pack 3 will indeed be coming to the PC – the team is hard at work on it and I'll let you know when it's in test.
As such, there will be a Patch 1.6 as well, which will address some additional game issues. When I have details on what else will be included in the patch I will be sure to post them.
We appreciate your patience as always, and look forward to its release as much as you do!