If 3 rak'ahs of a four-rak'ah Fard are already complete when congregation begins, what is the basic rule?

When Congregation Begins during an Individual Fard or Sunnah Prayer

• Complete the fourth rak'ah and finish the Fard.
• Discard all 3 rak'ahs.
• Give Salaam immediately after 3 as the normal rule.
• Turn the third rak'ah into the first.