I spent a good half an hour researching and trying to find the answer to your very question on my own GA-965P-DS3. I have three leads for you:
I found my revision number (rev 3.3) right next to the bottom most pci slot (I had a card in that slot and it was partially blocking the text (the number is between the slot and the back pannel).
It has been suggested elsewhere that Gigabyte often puts revision numbers near the RAM slots.
It has also been suggested elsewhere that the revision number may be between the PCI slots.