I have a couple of hard drives which do not have their own power supplies. Would it be possible to buy a USB Hub with it's own power supply and plug this into the Raspberry Pi 3? I use Kodi, will Kodi be able to find the harddrives via this USB Hub?
If you mean a powered USB hub, generally yes, they should work with no problems. The drives will appear as they normally do.
If you are worried, buy from somewhere that will take a return or exchange within a few days without a hassle -- but again, the chance of it not working is very slim.