Giovanni Battista Tiepolo
The Virgin Appears to SS. Agnes of Montepulciano, Rose of Lima, and Catherine of Siena

The Gesuati, Venice

Saints Rose and Catherine were Dominican tertiaries, St. Agnes a Dominican prioress. Here all three wear variations of the Dominican habit.

St. Catherine of Siena is the saint in white wearing a crown of thorns and holding a large crucifix, both of which will be attributes in some of her portraits. Standing beside her, St. Rose of Lima holds the Christ Child and her eponymous rose, both common features in her portraits.

Sitting on the right, St. Agnes of Montepulciano holds a small cross on a thin thread. The cross and the thread figure in an apparition recounted in her vita.

Detail: Agnes holding the cross and thread.

Read more about images of St. Agnes of Montepulciano.
Read more about images of St. Rose of Lima
Read more about images of St. Catherine of Siena.

Source: this page at Wikimedia Commons.
Many thanks to the correspondent who helped me with the identifications of these saints.