Lord Buddha appeared in this world with the sole purpose of benefiting all the sentient beings of triple realm. His appearance in this world is a very rare phenomena and is the outcome of the collective merits if his disciples and beings who are to be trained under him. Out of great compassion, Buddha Shakyamuni revealed many different means to attain enlightenment to win liberation from cycle of birth and death. Buddha Shakyamuni set forth three wheels of law to suit varying degrees of intelligence and receptivity.
At a place called Mrigadavana (Deer Park) near Varanasi, Shakyamuni Buddha turned the first wheel of Law which constituted the doctrines of four Nobel truths and Eight-fold paths and It is designated as sravakayana.
At a place called Gridha-Kuta hill (Vulture peak) near rajgir, Buddha turned the second wheel of law which constituted the doctrines of Emptiness and selflessness of the person and the phenomena. These doctrines are vividly preserved in prajnaparamita literature Vaipulya sutras. These approach was later known as Mahayana or the path of Bodhisattva.
At Vaisali, on the other hand, Buddha turned the third wheel which constituted the doctrines of Buddha nature as described in Tathagatagarbhasutra, Mahaparinirvanasutra and Dharanisvaraja Sutra.
At various places such as Dhanyakataka, Sriparvat, Kamakhya, Sirihatta, Purnagiri, Odiyana, etc. Buddha revealed the path of Mantra to his highly gifted disciples as shorter path to attain enlightenment. This approach was termed as Vajrayana which integrates all three vehicles.