I'm not sure about the home made formula recpie, perhaps there is some more information online that you can try to google.
As far as how much, and when to feed them - make sure you always feed them at a temperature of about 106 degrees. Any higher and it could burn the babys crop and the baby will die a painful death, or any cooler and the crop will slow down that will eventually lead to the shut down of the babys digestive tract and the baby will starve to death.
The brooder needs to be kept at the proper temperature, otherwise the chicks will get dehydrated if too hot, or the crop will slow down if its too cold. You said the birds were around 3 weeks old, but you must know the exact age of each bird. They do not all hatch at the same time (depending on how many eggs, they can be a week apart in age difference) and a 2 week old chick has a different feeding schedule then a 3 week old chick.
At 2 weeks old, the brooder needs to be about 90 degress F, at 3 weeks old about 82-85 degrees, at 4 weeks old about 80 degrees, and at 5 weeks old + room temperature.
Age 8-14 days old: The chick should eat every 4 hours, with a break during the night to allow the crop to empty completely. An example would be to feed at 7 am, 11am, 3pm, 7pm, 11pm. How much they eat will vary from chick to chick, probably around 2-3 cc's. Feed until the crop is nice and full but not tight. Over stretching the crop can lead to problems that kill the chick. At this age the formula should be like thin gravy.
Age 15-24 days - 7am, noon, 5pm, 11pm. Approximately 4-5 ccs.
Age 25-34 days - 7am, 5pm, 11pm. Approximately 7-12 ccs. You can offer weaning foods at this age. Scatter pellets, seeds, steamed veggies (cut in small pieces) and fruits (cut in small pieces) across the brooder floor. Also place millet spray on the brooder floor. Place a small shallow bowl of seed and pellet in one corner and they will eventually learn to eat from the dish. If they are starting to eat weaning foods well, you can offer a shallow bowl of water for them. Do not put too much water or they can drown. I like to use jar lids to offer the food in. At this age you can thicken the formula to creamed soup consistency.
Age 35-44 days. Feed at 7am, and 7pm. Remember if they are losing weight, supplement with an extra feeding in the afternoon. Only cut out meals when they maintain their weight with the meal being cut out. About 10 -12 ccs. Forumla should be the consistency of cake batter.
Age 45 days - weaning. Feed at 7pm. 10-12 ccs.
Remember to weigh the birds regularly throughout the entire process and if you notice the chicks start losing significant weight (its normal to lose a small amount during weaning) then they need to be supplemented with extra formula. Remember each chick is different and weans at a different time.
Also remember to clean the brooder at every feeding.
Also just to note, as the chicks get older they are less reluctant to eat from you. At 3 weeks old + you might have trouble feeding some of them, as they might refuse food. With chicks around 3 weeks old that refuse to eat, or are reluctant to eat you can expect it to take about a half hour (or longer) per chick to feed them.