Don't worry, they are like this because they are too young u said it they are only 3 weeks, when they are 1 and a half month this thing will go,so don't worry!!!!!!!!!

 Are you cleaning their bottles or syringes after each use? Is the milk that you are feeding them made specifically for them? It could be that they have colds.
When my cats took sick, I took 1 of them in, because I can't afford to take all 4 in, I told the vet to give me the medicine for the rest of them, because they were all sick in the same way.
You could do it this way too, just take one kitty in, and tell the vet to do the same thing, it will cost a little for the 1 kitty, rather than for all of them, follow all the directions to the letter to ensure that they all have a chance to recover. Hope this helps, good luck.

 When a kitten is breathing funny it is pretty serious.  If they are panting through the mouth rather than breathing through the nose, it is usually an indication that they have aspirated liquid into the lungs.  This can happen easily when they are being bottle fed or fed with a dropper.  Tiny babies can only swallow a tiny bit at a time.  You may have been feeding them too fast.  A vet can x-ray them to see if they have fluid in the lungs and would know what to do to fix it.  If you can't afford vet bills keep the kittens very warm (all of them).  Hold the two that are breathing funny between your hands with their heads pointed to the floor.  Gently turn them with their heads pointing up so that your enclosed hands look like you are praying and then swing them until their heads are pointing to the floor again.  Make certain you are holding them firmly and that their heads are being held securely.  This will help release any fluid from their lungs.  Try this three or four times and see if any fluid comes out of their nose or mouth.  Clean up any fluid around their noses.  Most fostered kittens and puppies that die, get too cold.  Keep a small space heater near their bed.  They can't regulate their body heat until they are over 5 weeks old.  The second reason for deaths in fostered babies is feeding them too much too fast and they aspirate the fluid or wet food into their lungs.  Make sure you are feeding them KMR (kitten replacement milk).  Good luck.