Short answers, pulled from the story.
The Pali Canon describes the threefold support as the Buddha, the Dharma, and the Sangha. The first jewel is Sakyamuni Buddha, the second is the spiritual doctrine of impermanence, and the third is the monastic order that preserves the teaching.
Mahayana texts like the Ratnagotravibhāga treatise explain the triple gem through the Dharmakaya where the Buddha represents peace and self-enlightened wisdom. This tradition includes innumerable Buddhas such as Amitabha and Vairocana while defining the Sangha as beings who realize the true luminous nature of the mind.
Huineng urged listeners to take refuge in the three jewels of their own essential nature rather than external figures. In this context, Buddha refers to awareness, Dharma refers to truth, and Sangha refers to purity found within the heart.
The Triratna symbol visually represents the Three Jewels on frieze sculptures at Sanchi from the second century BCE. It appears as a lotus flower within a circle surmounted by a diamond rod or vajra and serves as a decorative symbol on later gates at the stupa from the second century CE.
Historical coins of Buddhist kingdoms in the Indian subcontinent feature this symbol prominently including those issued by the Kuninda Kingdom in the first century BCE. Coins of Abdagases I of the Indo-Scythian and Vima Kadphises of the Kushan Empire also display it.