What is the significance of the three stars in Peter Paul Ruben's Saturn devouring a Son painting?
I'm afraid the stars do not have mythological significance. According to the listing for the painting on the Prado Museum website, the stars are probably the planet Saturn, whose rings were believed to be satellites at the time:
The stars in the firmament above the cruel old man remind us that Saturn is also the name given, back in antiquity, to the sixth planet in the Solar System that is visible to the naked eye. However, this is an odd arrangement: beside the main gleam of light are two smaller stars whose presence is not quite clear. It is now known that Saturn is surrounded by a ring that can only be seen with a telescope. When Galileo Galilei discovered this phenomenon in 1610, he believed that the ring was two satellites positioned on either side. Was perhaps Rubens familiar with this interpretation? Certainly, the painter followed all the advances of his age with extreme interest; however, this hypothesis would have to be examined at length in order to be given serious consideration.