poorly chick

i have been given 3 chicks to habd rear by my grandad, 2 of the chicks are 6 days old these hacthed in the nest with there mother but had to be taken away as she was not caring for them she had allready lost 3 to preditors, the other was hatched in a box on the aga 2 days later as she left it in the nest, it is alot smaller than the others as is not eating much, it can stand and will drink from the dish but it seem very wobbly and seems to just sleep alot, when you pick it up it up she doesnt open her eyes but does stuggle and cheep. any advice?

i have them under a heat lamp in the kicthen, but i have taken the smallest out as the 2 bigger kept standing on it, the bigger two are eating chick crumb and are very happy pecking and sracthing  around the pen