Contributions to a 401k are subject to social security and medicare tax, but not to ordinary income tax. So your W-2 Box 1 amount will be lower than your actual gross salary by the amount of your 401k contributions and any Sect. 125 or "cafeteria" plan benefits. Your Box 3 and Box 5 amounts will be your gross salary because the 401k and other benefits are subject to SS and Medicare tax. Box 12 shows the elective deferral amounts.
You can research all of this yourself by reading the instructions for Forms W-2 and W-3 at this link;
www.irs.gov/pub/irs-pdf/iw2w3_13.pdf
