Especially since they are 3 weeks old already, I would try the bigger bottle (i.e. preemie nurser mentioned earlier) as the tiny little bottles they sell for puppies are more for the newborn size. But they are probably big enough to lap up the milk from a saucer at this point. Have you tried that? If they won't lap milk or nurse from a bottle, then you will have to syringe in some milk. It is forcing but that is to make sure they survive. I had one that would not latch on, I had to use the syringe for a little over a week. Then finally he got the notion to suck! Thank goodness.
I want to repeat to use goat's milk. Mine did not do well on either of the prepared formulas I tried. But they thrived on goat's milk. I had to hand feed 5 pups from day 3 on through til they were weaned. It was a bittersweet journey -- sad as I had lost their momma but such a joy as all 5 made it fine.
Hang in there as next week you can get them to start eating food from a bowl and ease up a bit on bottle feeding. So, not too much longer.