Most rabbits deliver all of their babies within a few minutes, but sometimes extended labors happen. Angora breeders have told me does of their breed are notorious for delivering over the course of a couple of days. I have only seen this a few times; I can remember one doe that had three babies one day, and 3 more three days later. Most does won't breed when they are pregnant, but there are exceptions. Rabbit does sometimes get pregnant only on one side, and then breed again at a later date, and wind up carrying two separate pregnancies with two different due dates.
Most people have very poor luck trying to hand-feed baby rabbits; in almost all cases, you are far better off letting the doe do her job (I have had a few does that simply had no milk; obviously, their babies would starve, so that would be the exception). Some does don't seem to know that they need to nurse the babies once they are born; those does need to be taught their job. Some people will take the doe into their lap, lay her on her back, and put the babies on her, and some does will tolerate that. On the other hand, I had one doe that abandoned her babies after I tried that - she just couldn't stand that much interference. What has worked for me is putting the nest box in a carrier that is only slightly larger than the box, putting the doe in the box, and closing the lid of the carrier. With the doe stuck in the box, most of the time, the babies get fed with a minimum of struggle and stress. Most does that have needed this treatment catch on within a couple of sessions, and go on to care for their litters without any further issues.
The fact that the kit born solo is noticeably smaller than its siblings is intriguing. There are several possible reasons for that, from genetic issues to plain bad luck. What breed are we talking about here?
