If it's making a scab and nobody pulls the scab off, it will probably survive. Don't know what the string may be, but bunnies are pretty good about repairing themselves if given a chance.
Otherwise, if it's a structural defect and it's only three days old, better that it go quickly than linger for awhile before it goes. If things aren't born right, saving them may not be the best idea. Some defects can be genetic and not something you'd want in further generations.
Are there pictures? Most umbilical cord scabs are really tiny things and I've never known of any strings attached. If it's an open stomach wound of some sort, then put the antibiotic on it, let it scab over and hope for the best. At three days old, I don't know if it would survive any injections of antibiotics nor would that be good for it's system, one would think since it should be setting up the gut biota at this point? If it gets an internal infection, well, that'd not be good but let's not look for problems.
What kind of rabbit is it? I'm guessing it's not meat rabbits?
