My employer entered 3 dependents on my W-2 form when I am single and on my original W-2 I did not claim any dependents.

When you file your taxes you will get a refund. Have your employer fix it for next year.

W-2 does not show dependents. You fill out and give to your employer a W-4. If the current W-4 is incorrect, then fix it. You cannot change the amount withheld for 2013 at this point. You have to file your tax return and if you owe money send a check with the return. I always think it is best to have as close to the correct tax withheld so that you get no refund or a very small refund. Otherwise you have given an interest free loan to the government, instead of having the benefit of the use of that money during the year.

Mr. Wagner gave you good advice. On the chance that the employer may have failed to account property to the IRS, you can request from IRS on its toll free line a transcript of your account for 2013 to determine how much tax is credited to your account. If the amount is short of what you think proper, then discuss it with an IRS representative and keep a good diary of what you do, when you do it, who you talk with and what is discussed.