Quotes from Treatise on the Great Perfection of Wisdom, Vol. 5
(Mahāprajñāpāramitopadeśa)
"The three concentrations (samādhi) are those of emptiness (śūnyatā), wishlessness (apraṇihita) and signlessness (ānimitta)."
Excerpts from Overview of Three-Vehicle Bodhi, Vol. 1
“Why are the Three Dharma Seals also called the three samadhis (samadhi of emptiness, wishlessness, and signlessness)? This has something to do with the notion of cultivation and true realization. At the stage of learning and trying to understand the connotations of the terms, we call them the Three Dharma Seals. However, when we have realized the Three Dharma Seals, our wisdom will be brought forth and the Three Dharma Seals will then be called the three samadhis. This is because we will gain a decisive and unwavering understanding of the whole dharma realm after we realize the Three Dharma Seals. Our understanding of it will become unshakable as a result. Therefore, we will be able to attain liberation. In this sense, we can be said to have attained samadhi, and the realization of the Three Dharma Seals leads to the three samadhis.”
Overview of Three-Vehicle Bodhi: Sound-Hearer Bodhi 1, p. 61.Wholesomevision Publication