If you want to use all 3 drives at the same time you will either need to have a powered usb hub or one of those rather expensive usb storage devices that support raid/stripe on 2 or more disks (like the Lacie 2big Thunderbolt
http://www.lacie.com/products/range.htm?id=10061 ).
If on the other hand you want to plug in one at a time it's just a matter of choosing a convenient way of flipping them around that suits your budget.
Schneidz's solution id pretty good, I've a similar adapter that I find very handy, it came with tan external power supply supplement for driving full power 3.5" drives.