It will depend on what kinds of plants you want to grow. Some require more light than others.
With the 5g, you will need more wpg to get the necessary growth. I would count 10 watts as low light. Maybe go for 15 or 20 watts if you want a higher light tank, if it's physically possible (depending on dimensions of tank etc.).
For the 20 and 30 gallon tanks, I would aim for 2.5 to 3 wpg. So, around 50 or 60 watts on the 20g and 90 to 1110 watts on the 35g tank.
You can go for less if you want low-light plants, or have one as a low, one as a medium and one as a high-light tank... or whatever.