Update 3 is now available for Revit 2013 products.
For Autodesk Revit Architecture, Autodesk Revit Structure, and Autodesk Revit MEP, you can find the update in the Data & Downloads > Updates section of each product page.
The update for Autodesk Revit 2013 can be found in the Design & Creation Suites Support > Data & Downloads > Updates & Service Packs section.
Here are the direct links:
Installation ReadMe files and Update Enhancement PDFs are available at each of these locations as well.