well at 3 weeks i usaully start feeding nutrients.
they do look under nourished
and the yellowing could be a magnesium deficiency, curable with epsom salts which can be aquired at ya local chemist
mix about half a teaspoon of epsom to 1 litre of water, making sure that it is completly dissolved ,and water like normal with the solution
i also spray down the leafs when the lights go off and this seems to help
hope this helps m8
cheers all