The studies of Neandertal mtDNA do not show that Neandertals did not or could not interbreed with modern humans. However, the lack of diversity in Neandertal mtDNA sequences, combined with the large differences between Neandertal and modern human mtDNA, strongly suggest that Neandertals and modern humans developed separately, and did not form part of a single large interbreeding population. The Neandertal mtDNA studies will strengthen the arguments of those scientists who claim that Neandertals should be considered a separate species which did not significantly contribute to the modern gene pool.
They come to this conclusion after looking at the mtDNA of THREE Neandertals?
Yes. Although it's long been thought that Neanderthals were not direct ancestors of modern humans anyway, but evidence other than anatomical evidence has been hard to come by until now. It's not definitive, given that the DNA analysis is not perfect, but it's another piece of evidence that Neanderthals and modern humans share a common ancestor rather than one being descended from the other.
IOW, this is hardly an unexpected result.