I also didn't feel that 3.5 oz is too small in a kitten. If your scales can weigh by grams, it is sometimes easier to see the weight gain or loss. A kitten should gain around 10-15 grams a day. Try and keep a journal and weigh at the same time each day for more accuracy.
Is the hair trimmed away from the nipples? In medium or long hair cats that can sometimes get in the way of the kittens finding their own teat.
You may also contact your vet about something to help the Mother's milk supply. He may suggest certain herbs that will enhance her production of milk, or suggest a higher calorie food, such as feeding a canned food such as Hills A/D, along with a kitten kibble to help with calories she loses nursing a large litter.
A healthy newborn kitten is either nursing or sleeping. If one moves too far away from the others, move it back to the siblings to help it stay warm. A kitten will digest food at a quicker, safer rate, when kept warm.
If you do end up deciding to supplement, make sure the KMR is warm, and that the baby is warm as well, (never feed a chilled kitten, it can cause death). It is also easy to overfeed, I believe some have already linked sites, that should say the amount to feed at certain ages. Also, if a kitten has trouble bottle feeding, your vet can show you how to tube feeding, which can be a lot less stressful on you and the kitten.
Good luck with all the babies!!