DoD's website is in the public domain (unless the material is marked otherwise). That's because under U.S. law, any works produced by federal government employees within the course of employment is considered to be in the public domain. That doesn't mean the government has to give you the works for free -- for example, the DoD can charge for duplication and handling charges. The government can also establish restrictions on commercial uses. Assuming you meet the standards for your commercial use, there doesn't seem to be anything prohibiting you from your three case scenarios. The DoD often provides the name of the military photographer but we're not seeing any requirement that attribution be provided with each use. That's up to you.
But wait there's more ... The rules regarding accessing and acquiring DoD imagery are about to change. According to announcements made last month, a private company has been awarded a contract to digitize and license DoD content. Here's the DoD's statement of objectives for the project and here's an article explaining how it works. In short, accessing DoD works may involve more fees and more licensing restrictions.