I would qualify Optikus's reply : manual focussing on an slr is much more difficult with wide angle lenses than with longer focal lenses. That's because things don't "snap in focus" in the same way, due to the wider dof. You may see things in focus on your viewscreen, which provides a small enlargment of your picture, but when printed at a significant size, you find out that your focus plane is not where you expected it to be.
Even if you've manual focused with an SLR before it may not be like MFing on a DSLR. Keep in mind that MF only SLRs had good focusing screens that sacrificed brightness for accuracy. Things like split prisms are extremely useful.
New SLRs don't have these focusing screens and thus it is extremely hard to nail manual focus. You could always use liveview to MF in 5 or 10x, but this is slower.
Expect to upgrade the focusing screen (not easy) in your 5D3 to maximize MF lenses.