A sloppy mix of chicken, rice and formula probably would be the easiest to start with. Expect them to walk in their food at first. Kittens are messy when they first start to eat on their own. They might not be quite ready to eat on their own, but at 3 1/2 weeks, it's worth a try. You possibly will have to try feeding them off a finger at first, then lead them to the food dish on subsequent attempts.
One of the cats I have now started eating on her own somewhere about the age yours are (don't really know how old she was--mama abandoned her under my car).
One of the cats I have now started eating on her own somewhere about the age yours are (don't really know how old she was--mama abandoned her under my car).