To lose Hazard would be disastrous. To lose either of the other two would also be really bad.
Also, I don't think Courtois could be replaced?! Certainly not with ease. Who's out there that's available at his level? De Gea and Neuer is not even worth a debate, Lloris wouldn't venture across London. Before the "Chelsea are a defensive team so their keeper doesn't need to be world class" lazy comments, Courtois has pulled off some REALLY big saves in big moments of matches this season, and in his own right, is just as responsible as Kante, Hazard or Costa etc is for where Chelsea are. The guy is truly world class.
As seen with City and Liverpool, the grass is not always greener, Bravo and Karius not the keepers they thought they signed (at least not yet in the latter case), so Courtois could take some replacing.