As far as the chick that is already in Crop-Stasis, if he's still alive,]you need to buy a bottle of unflavored Pedialyte (as he will dehydrate extremely fast at his size if he misses even one feeding, so he needs the electrolytes, as do the other 2 chicks), as well as a bottle of Avian Probiotic Powder at Petco/Petsmart (they both sell bottles of Qwiko Avian Probiotics for $10), OR a much better option that will save him if he's still alive at this point, but even if he's not it will save the other 2 chicks as long as you get the temperature corrected immediately, Walmart sells a Probiotic Supplement for humans that costs only $5.96 a bottle (I'm close, it's right under $6 a bottle), made by Spring Valley supplements/vitamins company (Walmart sells all of their supplements and vitamins in the human vitamin aisle, the Spring Valley bottles are dark green and have bright yellow caps and labels), that is called "Multi-Enzyme Digestive Formulawith Probiotics"...The reason that this is a much better option for this issue is because it not only contains multiple strains of Probiotics, which will help to add a ton of beneficial-bacteria to their GI Tracts, which will immediately start to get the yeast under control, but it also contains ALL of the naturally-occuring Digestive-Enzymes that are normally found in their Crops and throughout the rest of the GI Tracts, and they will help to immediately get their Crops moving again, or in the case of the other 2 chicks, if they haven't reached the Slow-Crop or Crop-Stasis stage yet, this will keep it from happening. I always keep Probiotics as well as Digestive-Enzymes on-hand whenever I'm hand-feeding a clutch, because even though my formula temperature and my Brooder temperature is always correct, it's still quite possible for chicks who are only 2 or 3 weeks-old to develop yeast infections or simply develop Slow-Crop or Crop-Stasis for some other reason...
What you want to do is crush one of the Spring Valley "Multi-Enzyme Digestive Formula with Probiotics" tablets into as fine a powder as you can get it (they crush easily) and then put the powder into a little baggie or small, plastic container so that you'll have it on-hand...Then you need to get a clean, microwave-safe container and pour 1 cup of unflavored Pedialyte (use a measuring cup) in the microwave-safe container...Then add 1 Teaspoon of the powdered Digestive-Enzymes/Probiotics tablet to the 1 cup of unflavored Pedialyte, and then mix it very, very well, until all of the powder is dissolved into the Pedialyte...Then you want to microwave this for 30 seconds, take it out, and then mix it up again very well...You'll have to use the cooking/candy thermometer to make sure that the temperature of this is also between 104 and 110 degrees F, and you're going to give this to each chick in-place of one of their hand-feedings of formula...You cannot just keep putting formula on top of the already-rotting formula that is sitting inside of the sick chick's Crop, and this mixture of the Digestive-Enzymes and Probiotics will typically start their Crops clearing within about 15-20 minutes of them getting it. They may or may not take it, sometimes getting a flavored Pedialyte like Grape, Cherry, Fruit Punch, etc. works better to get them to take it, but the chick who is already in Crop-Stasis should be very thirsty and hopefully you can get them all to take a good amount of the mix...I'd actually recommend getting a flavored Pedialyte instead of the unflavored now that I think about it, because they should be willing to take that more readily...You want to give them as much of this as they will willingly take OR 1cc/ml, whichever comes first...The you wait and watch the Crop to see if it is starting to empty...YOU CANNOT FEED THE SICK CHICK ANYMORE FORMULA UNTIL YOU SEE THEIR CROP START EMPTYING, BECAUSE YOU'LL JUST MAKE IT WORSE. So if you give the sick chick at least 1cc/ml of the Pedialyte/Digestive Enzyme/Probiotic mixture and it starts to empty but doesn't empty completely, then give that chick another feeding of the mixture until the Crop completely empties, and then you can give him a formula-feeding that contains some Apple Cider Vinegar mixed into it from now forward.
If the Crops of the other 2 chicks are still emptying fine, I would still give them one dose of the Pedialyte/Digestive Enzymes/Probiotic mix, let it work it's way through and wait for their Crops to again empty, then you can go back to feeding them the hand-feeding formula, but I would add some Apple-Cider Vinegar to the formula just in-case...I don't know how much formula you are mixing-up at one time, but you want to add 1 drop of Apple-Cider Vinegar to every 1ml/cc of hand-feeding formula they eat (you cannot keep already-mixed formula for more than 12-hours, even in the fridge, as it starts growing an enormous amount of harmful-Bacteria as soon as it cools, so you must throw any you have left out after 12-hours since you first mixed it up and make new)...I would not feed the chick with the Crop-Stasis anything but the Pedialyte/Digestive Enzyme/Probiotic mix for the rest of today/tonight and then tomorrow morning you could try a little bit of formula at the correct 104-110 degrees F, like maybe give him 1ml/cc of it first thing tomorrow morning that has 1 drop of Apple Cider Vinegar, make sure his Crop empties normally and he only has a small bit left in the bottom of his Crop after 2-3 hours, and then if it does then go back to normal hand-feeding times and amounts with 1 drop of ACV added for each ml/cc of formula...
