help needed urgent

Hi,All, 3livers , in flower 2weeks, 2 look quite healthy the odd yellow leaf here and there but the 3rd dos'nt look as healthy although bigger, but definatly weaker, i've had this yellow leaf problem for weeks now, apart from when i flushed before flippng when they all was lush green , it is driving me mad, and feel like terminating the yellow one, any advice more than welcome