What are the three reasons Chaucer's work is considered a masterpiece and important work in literary history?

There are many answers to this question, but since you asked for three I'll offer the following:

1. History/Culture

2. Faith

3. Identity

The tales speak in wonderful detail about the historical ethos of fourteenth century England. They also highlight in powerful precision some key themes in the Christian tradition. Lastly, the rich diversity of character in the story seems to foreshadow the modern age's emphasis on individual identity and choice. These, among others, make it a great collection of stories.