When equanimously observing, does progress come from seeing how craving is painful or from seeing that sensations aren't worth craving due to being inconstant or from seeing that there is no self?

It is literally via the 3 Characteristics, these are the doors to liberation

One who, at the stage of advanced insight, contemplated things as not-self, acquires the emptiness concentration on arriving at the path and fruit (because he has seen things as empty of self); one who contemplates things as impermanent acquires the signless concentration (because he has seen things through the “sign of permanence”); one who contemplates things as suffering acquires the undirected concentration (because he has no leaning to things seen as painful)

It comes from conditioning the mind not to construe phenomena as satisfying, permanent or self, in an expanding range of circumstances.