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Hi there!At 3.5 weeks of age, kittens should be fed milk replacer, which sounds like you're already doing. I would use a bottle/nipple combo though instead of a syringe because feeding by syringe increases the risk of aspiration (inhalation of milk into the lungs). There are many types of bottles for kittens and I would choose a set with different nipples so that they could "grow" with the kittens. How much and how often to feed is based on age/weight and you can find more information on this here: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=576
After each meal, you should stimulate the kittens to urinate/defecate; you can do this by taking a cotton ball, running under tap water and gently stroking the kittens' vaginal/anal area over a skin, etc., so that they could eliminate.
At the age of 5-6 weeks, I would start offering a mix of water and kitten canned food in shallow dishes and water separately to drink also from a shallow dish. Over a period of 1-2 weeks, the kittens should transition to the kitten food fully and you could at that point start adding dry food.Hope this helps!