No a garnishment is not the same as tax withheld. Tax withheld is in box 6 and is for current years Federal Income taxes with held for 2017 tax liability
A description of an amount in Box 3 includes a garnishment or Levy
A garnishment or levy is for overdue federal taxes, or other specific over due debts. Per the Social Security Administration:Treasury benefit payment offset, garnishment, and/or tax levy – If we show an amount in this legend, either the Treasury Department applied that amount to a debt or debts owed to other Federal agencies; benefits were withheld to pay child support, alimony, or court ordered victim restitution; and/or benefits were withheld to pay debts to the IRS . Direct inquiries about what type of offset is involved to SSA at 1-800-772-1213. Direct questions related to federal taxes to the IRS at 1-800-829-7650.
Or see this website. SSA-1099 Garnishment.