Even if you are not self-employed, the amount listed in Box 3 could be subject to self-employment tax (which will end up being your portion of the Social Security and Medicare on this income).
The IRS classifies supplemental wages as bonuses, severance pay, taxable fringe benefits, vacation pay, back pay, and overtime. Supplemental pay is anything other than your regular pay.
If supplemental pay is reported on a 1099-MISC in Box 3, then you will still need to pay the uncollected Social Security and Medicare tax, as well as the income tax. This could be why it is populating under your Schedule C and adding self-employment tax to your tax return.
Typically, supplemental pay is added to a W-2. However, there are different withholding methods for supplemental wages that the employer can consider, which are discussed in IRS Publication 15.
Per the IRS, regardless of the method employers use to withhold income tax on supplemental wages, supplemental wages are subject to Social Security and Medicare taxes.