What do Thomas Merton, Viktor Frankl, and John Henry Newman share in common in their outlook on the meaning of being human and what it is to live a life of meaning and purpose?

Thomas Merton's conception of what it means to be human was informed by his life as a Catholic Monk and his openness to a huge array of Eastern religions and lines of spiritual thought. Though he identified as Catholic, he internalized the ideas of other spiritualities, believing that they each explored unique "depths" within the human experience of the universe. In the same vein, he rallied against his own church's dogmatic promotion of formal Cartesian concepts as if they were ends in themselves.
Viktor Frankl approached the search for human meaning primarily through the lens of a psychology strongly informed by contemporary European philosophy. He viewed human meaning as something that a given individual has a natural will to aggregate as they suffer through time. He is similar to Merton in that he resisted the promotion of concepts that regulate experience, instead endorsing the individual search for meaning and the fundamental process of deindividuation that underlies meaning-making.
John Henry Newman's views on human meaning and purpose were the most institutional and conservative. He believed in a "Judeo-Christian revelation" that took primacy over all other revelations one can make about the world. However, he too believed in the reality and significance of a search for meaning outside of Christian conceptions, and he also held an idealized view of the world free from symbols that exert oppressive or confusing ideological power. He rationalized his simultaneous recognition of a valid Christian and valid non-Christian search for meaning in two concepts he termed "natural religion" and "revealed religion." The former is the endless and undirected set of revelations one can make about the world without the influence of Christianity; the latter is a more directed, controlled, and ultimate kind of revelation that proceeds from the internalization of Judeo-Christian doctrine.
All three thinkers held what the contemporary philosopher Richard Rorty would call a liberal ironist view of human experience and purpose, departing from the central axiom that human understanding is contingent, incomplete, and evolving; this understanding is limited by humans' inability (at least in any mortal state) to totally translate reality into intelligible symbolic chunks.