Ok, the last one is just smaller than the other ones, so you should be fine (i.e. by now you should have oe_sources.taraa, oe_sources.tarab and oe_sources.tarac with sizes 800,800 and 584 megaytes).
Put them somewhere on a linux box, and do "cat oe_sources.taraa oe_sources.tarab oe_sources.tarac > oe_sources.tar" and you should end up with a fine tarball. Let me know if it works (I guess theres a similar tool to cat on Windows, but I wouldn't know...).
Cheers
Philipp