If you are drinking 3 litres a day that should be enough depending upon your physical activity. The colour of your pee is a good indicator of your hydration. Your AFI would normally start to fall at about 34-35 weeks. Low AFI can indicate an abnormality with your baby where the tube that connects the mouth to the stomach is not formed correctly however this is easy to detect by ultrasound and I would think that your doctor checked for this. Aside from this I think that you just have a low AFI and would expect your doctor to suggest at least monthly scans until birth.