An amount appearing in box 3 of IRS Form SSA-1099 Social Security Benefit Statement would not be entered in the TaxAct program in box 5. Box 5 on the form is the Net Benefits for the current year and has already taken the amount from box 3 into account.
An example of Form SSA-1099 is available in IRS Publication 915 Social Security and Equivalent Railroad Retirement Benefits, page 20.