fostering 3 week old kittens

I am fostering 3-week olds
they are eating every 3 hours and seem to be doing well. i have o nly o ne question..they pee on there own but when I see they are uncomfortable..I help them along by rubbing them on there pee-pees and they go alot . they did not poop yet ... how do I help them with that?
I have a tiny litter box in there cage and they keep going to it scrathing but no poop...I got them yesterday... Any ideas on how I can help them?
Also, i read that overfeeding them is really bad.. but they act like they are so hungry...I do not have a scale small enough to check there weight but i think they are eating a little more than they should be..they will consume 2-3 tablespoons between them and in 10 min cry for more then i give just a little bit and then they are content. I am open for any sugestions

what i did as stimulate them, then put them in the litterbox, that seems to work.

on the over feeding, if they still act hungry after 10 min, try givin them some water instead of formula, that might help, or a mix of 10% formula, 90% water. i have seen kittens go 24 hours between poops, especially after stress like being moved like yours were.