The saints are, from left to right, Martin (identified by the beggar in the background), Bernardino (the donor's patron) and Donatian. The donor was secretary to the Chapter of Saint Donatian's (Bruges) from 1489, becoming canon in 1498.
The painting may have originated as the left wing of a diptych for the altar of Saints John and Mary Magdalene there, commissioned in 1501. The right wing is probably the 'Crucifixion' in the Gemäldegalerie, Berlin.