What are the 3 main reasons the French and Indian war started?

The French and Indian War (1754-1763) should be thought of as part of a worldwide struggle between the British and French.  That conflict is better known as the Seven Years War.

The main cause of the French and Indian War is the superpower competition between Britain and France.  These two countries were the most powerful in the world and competed for dominance around the globe.

Within this overall cause, there were reasons specific to North America that helped cause the war.

1. The desire on the part of both countries to control the lucrative North American fur trade.

2. The fact that the expansion of the British colonies in North America were threatening French territory and their trade routes.

These three factors (which can be seen as two subfactors of a main cause) were instrumental in causing the French and Indian War.