Act I lasts from the beginning of the story until the moment when the characters agree to the Host's proposal of a tale-telling competition.
When the characters take oaths to engage in a tale-telling competition and abide by the Host's judgment and authority, they are fully committed to the action that follows. Chaucer's description of meeting the pilgrims at the tavern in Southwark and becoming one of their number, as well as the portraits that follow, establish the characters and set the stage for the fun that ensues.
Act II lasts for the entire pilgrimage, not to conclude until the last pilgrim has told his last tale.
Since we can't know who the most successful tale-teller is until the last pilgrim has told his last tale, Act II lasts for the entire length of the pilgrimage. Only then, when all the tales have been told, can the story begin to make its way toward resolution.
The Canterbury Tales have no third act, because they're not finished.
The third act would be everything that happens after the last pilgrim tells his last tale: the revelation of the competition's winner, maybe some discussion about what makes for a "good" tale, perhaps the celebratory banquet. But the pilgrims don't reach Canterbury, let alone arrive back at the tavern, so the story doesn't even begin to approach Act III.