No, a 403(b) is not an IRA. Enter amounts shown in box 12 of your W-2 only in box 12 of TurboTax's W-2 form, nowhere else in TurboTax. Your employer has already reduced the amount in box 1 of your W-2 by the amount of your elective deferrals to the 403(b), so you don't get to deduct them again.