There are a probably a few ways to answer that--and hopefully you'll get some good responses. The most important thing I can think of is to answer what kind of mobile experience does your client need to present? A lot can be done with responsive sites, particularly for businesses that want a mobile presence but don't need to have separate mobile-specific content. The mobile theme switcher in c5 can also provide a quick solution without having to use a separate browser detection script to direct to another mobile-specific theme. There isn't much in the way of c5 themes designed specifically for mobile devices at this point, so some CSS customization might be in order to match the look of the default mobile theme to the website's. To get the fastest performance from a mobile site, you are still probably better off with a straight HTML site (no CMS overhead), but then you have to manage 2 sets of content.

So probably you'll just need to first determine your client's mobile needs and learn what the mobile needs/demands are of the audience that they are trying to reach. Those findings should dictate the technical and design solutions rather than the other way around--even for a 3-page site. Love your avatar.