The third visuddhi of the Satta Visuddhi is "ditthi visuddhi". But what is the exact meaning of it ?
Purificatiin of view:
'The divine eye is purified since it is a cause for Purification of View (diṭṭhi-visuddhi), owing to seeing passing away and reappearance of beings. One who sees only passing away and not reappearance assumes the annihilation view (uccheda-diṭṭhi); and one who sees only reappearance and not passing away assumes the view that new beings appear (nava-satta-pātubhava-diṭṭhi). But since one who sees both outstrips that twofold [false] view, that vision of his is therefore a cause for Purification of View. And the Buddha's sons see both of these.' (PsA 259; Vism 423)
Wisdom and the Seventy-Three Kinds of Mundane and Supramundane Knowledge, given by Ven. Bhikkhu Ñāṇadassana
The Nyanatiloka Buddhist Dictionary defines it as "purification of insight", though this book on meditation gets a lot more in depth on what that actually means in practice (look at chapter 3). I'm finding it difficult to summarize as it's a bit dense for a beginner like me, but you might be able to glean more meaning from it.