Sorry for the loss :( I would get the lone chick a couple little chick friends - they do not do well alone as you can already tell. Don't beat yourself up but learn from this and raise some healthy, thriving chicks! There is a slew of info on this site on what you will need to raise days old...
Get rid of the corn and put them on a starter/grower feed. It looks like watery, loose stools around as well. At this point, give it electrolytes along with proper feed and hopefully it perks back up for you!
ETA: what are you using for warmth? They should have a heat plate or a heat lamp and...