?
Although the VS2015 release is around the corner, we don't believe the v3 API will be ready/stable for public consumption yet. There are no currently no API docs, and the codebase is still moving too much to try to guess what will it will be. We reverse engineered what we could with NuGet3RC2, and it seems to use a blend of v2/v3 API calls.
Once the API is ready, we will likely use it to implement the "nuget.org cache" feature that's on our roadmap.
That said, the NuGet team will need to (and plans to) support the v2 API in both their client and server components for many years to come, so we're in no rush. The v3 API will primarily offer performance improvements for reaching NuGet.org, but ProGet users aren't really impacted by performance problems anyway, due to local server caching.