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Line 3 is the amount of your income that is subject to Social Security tax.  Line 5 is the amount of your income that is subject to Medicare tax.  In many cases, lines 3 and 5 are the same as line 1 (your gross income).  They may be different if you have certain payroll deductions.

Those are the amounts of your wages on which social security and medicare are collected.  Just like Box 1 shows the amount of your wages on which Federal income tax is collected.Box 3 and 5 may or may not be equal to box 1 if, for example, your have medical insurance deducted from your line 1 amount, or have contributions to a retirement plan like a 401k deducted.