The notion of the three turnings of the wheel of doctrine (dharma-cakra) was probably first articulated in the Discourse Explaining the Thought (Saṃdhinirmocana-sūtra), the most important scriptural source for the Yogācāra school of Indian Buddhism.
In the seventh chapter, the Buddha declares that he presented certain doctrinal teachings in three cycles, or wheels. The first wheel contains discussions of core doctrines such as the four noble truths (ārya-satya) and dependent arising (pratītya-samutpāda); this is the Lesser Vehicle (Hīnayāna), which is surpassed by the superior teachings of the second wheel. The second wheel is the Perfection of Wisdom (prajñā-pāramitā) discourses, which analyze previous doctrines and the phenomena of the universe and declare them to be empty (śūnya) of inherent existence. In the third wheel of doctrine, the Buddha provides further clarification regarding what is and is not being negated by the second wheel teachings.
The Discourse Explaining the Thought does not specifically list particular doctrines as belonging to the third wheel, but the overall context indicates that the reader should assume these to be the doctrinal formulations of the sutra. The third wheel is declared to be the final thought of the Buddha, but it is reserved for a small elite. The Tibetan scholar Tsong-kha-pa is probably correct in asserting that only certain teachings fall within the purview of the three wheels. Regulations regarding monastic dress and conduct, for example, do not appear to fit into this classification.
The notion of three wheels of doctrine is probably linked to the title Discourse Turning the Wheel of Doctrine (Pāli, Dhammacakka-pavattana-sutta), which, according to tradition was the first sermon taught by the Buddha. The Perfection of Wisdom discourses were presented as superseding this and other Hīnayāna teachings. The Discourse Explaining the Thought implicitly invokes this notion of successive cycles of instruction delivered for progressively more advanced audiences. (In Sanskrit, the term is tri-dharma-cakra.)
The most important classical statement of the three turnings of the wheel of doctrine is in chapter 7 of the Discourse Explaining the Thought. A detailed discussion of this concept is in Powers 1993, chapters 5 and 6. Blumenthal 2008 provides a useful general introduction to the three wheels. While many students may first encounter the Wikipedia entry on the three wheels early in their search, the article contains a number of significant inaccuracies and should be avoided.
What Is and Isn’t Yogācāra is a short overview of Yogācāra in general that has a brief but useful summary of the three wheels doctrine. The Yogācāra Buddhism Research Association website is the best online source for information regarding Yogācāra; it contains several useful articles, bibliographical information, and links to pages for major Yogācāra thinkers. It also links to the Digital Dictionary of Buddhism, which has a wealth of material pertinent to the study of Yogācāra doctrines.
Indo-Tibetan Buddhism: Indian Buddhists and Their Tibetan Successors by Snellgrove (1987), intended mainly for specialists, has a good discussion of the three turnings of the wheel of doctrine in India and Tibet. The Inception of Yogācāra-Vijñānavāda by Buescher (2008), provides a good overview of the early origins of Yogācāra, but to date there is no comprehensive study of the doctrines, practices, texts, and philosophers of this tradition.