Now, congratulations on their poo — toothpaste consistency and yellowish brown is perfect poo for bottle-fed babies.
If the vet said one teaspoon powder to two parts water, she meant one teaspoon powder, two teaspoons water. If you were making up a litre, it would be one litre powder to two litres water. Or one cup powder to two cups water. The ratio, in other words, is 1:2; twice as much water as powder, however much powder you use.
Of course you don’t want baby kittens dying on your watch. However, the very sad fact is, and this is true for the US and Europe and Australia and everywhere, abandoned babies often do not do well. Up to 40% of them die without their mums. On the bright side, this also means that 60% of them live and if yours are three weeks old, then they stand a fighting chance. The trouble is, their mum probably had bad nutrition during her pregnancy, she probably was unvaccinated so any viruses she had, she will have passed on to her kittens, she may have been injured or gone through trauma during her pregnancy and she may have abandoned her babies because she knew there was something wrong with them. And when kittens are this tiny, there is not a great deal vets can give in the way of treatment — for example, antibiotics can do as much harm as good to a tiny kitten’s digestive system. So you must prepare yourself for the small chance that they will not make it to be healthy sturdy kittens. The vet wanted you to know that if this happens, it is IN NO WAY your fault — you are giving them a fighting chance and without you they would not even have that!
It’s a good idea to weigh them if you can, just to keep tabs on their progress — basically, you want to know that they are steadily gaining weight week by week, not standing still or worse, losing weight. It will give you a good idea of their progress and how they’re holding up.
They are suckling on each other because they don’t have their mum to suckle on any more. It’s good that it’s not on their genitals but it’s still not ideal — if they keep doing it, it can cause bruising and infection. You may need to put rolled up towels between them in the nest or, if that doesn’t work, separate them. But put some stuffed animals in their nest too and see if they can suck and knead on those. An unwashed woollen sweater of yours (preferably mohair if you have it) so it smells of you will hopefully also get them kneading and suckling on that rather than on each other.
ANything else, just ask, we are not vets but there are some very experienced foster kitty mums on here and they are wonderful at helping.