Why didn't the Prophet fight his enemies during the early years of Islam?

When the Prophet Muhammad (PBUH) became prophet, he did not actively fight the Meccans until he immigrated to Medina after three years. Afterwards, he had multiple wars with the Meccans (e.g. the Battle of Badr , the battle of Uhud and the Battle of the Trench ). Why did the Prophet wait three years to battle them, while during this time the Meccans were hostile to the Prophet and the Muslims and killed many of them during these three years?