The 3rd cup of wine is filled, but not yet drunk, instead Birkat Hamazon is recited - a blessing after a meal with bread.
Immediately after this, a blessing over the wine is recited and the 3rd cup is drunk.  


Baruch atah, Adonai Eloheinu, Melech haolam, borei p’ri hagafen

Blessed are You, Adonai our God, Ruler of the world, Creator of the fruit of the vine.