1. That particular mistake is "easy" to aquire. It happened independently in all three species 'cause that gene just mutates a lot. It is well known that some mutations occur more often than others. Something in the DNA structure is not well protected on that gene, so the same mutation will occure in many species.
If it does in species that don't eat a lot of fruit, the individual dies- that would be the only reason it does not show up in other species (if it doesn't!. I would not be suprised to see the same mutation in fruit eating monkies but not leaf or seed eating monkies.)
2. There was wholesale 'borrowing' of genetic code to make man. Some of the code used was from a chimpanzee. Not all that different than what I used to do when creating computer code. Leaving the extra stuff in does not hurt a bit, but my new program could never have 'evolved' from my previous one without my intelligent design efforts.
3. It may not be the same gene/mutation at all. A while back they made the astounding announcement that bacteria shared exact duplicates of hundreds of genes that humans have, but not most things 'inbetween' us and bacteria. THAT was seen as explosive evidence for DESIGN and the evos were scrambling for other explanations that seemed as implausable as my 1 and 2 no doubt seem to you. Later, they said they were wrong, it was more like 40 genes. Still astounding, but also amazing about how wrong they were at first.
I wonder if it really IS the same mutation, can they demonstrate a 'reverse' mutation that will restore the genes the ability to make vitaman C? Can they produce a chimp that can make its own vitamen C? If not, if they can't even undo a change for one tiny gene, what are the odds all those gene changes reqired for the man-chimp transition could occur in the time allowed?
That brings up another point. If evolution is so powerful that it can add info, why haven't we gotten that gene back? We are no longer mostly fruit eaters, as we were in the Garden. The plains Indians did not have access to vitamen C. A lot of peoples didn't. Wouldn't the ability to make vitamen C be a huge advantage for such peoples? Why did they never aquire the 'reverse mutation' that would restore functionality to this gene? Why don't we have Eskimoes that can make C while most of the rest of us still can't?
If evolution is only the loss or degradation of functionalty, then that argues for ID. An intial large amount of info slowly degrades over time as mutations are aquired.