What BookJunkieLI said.
Also if your going to be using 3 laptops in your household, then having your external hard drive network attached would make sense in a lot of ways. Many ways to acheive this, maybe next time you upgrade your router, you get one with a usb port...
For now though, I think it would be using an sd card/stick and as suggested, either just the library on the stick and calibre installed on all laptops, or portable calibre and the library on the stick.