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, on page 20.
Note that any link in the information above is updated each year automatically and will take you to the most recent version of the document at the time it is accessed.