as RealBeast stated, they would be compatible. The only downside would be loss of data transfer speed since USB 3 is faster than USB 2, thus your USB 3 HDD would operate at USB 2 speeds when connected to a USB 2 port. Not that big a deal though.
The transfer speed of your USB will operate at the slowest speed available (either from peripheral or computer). You can plug 2.0 in 3.0 and vice versa but it will only operate at 2.0 speeds (because it's the slowest).
Also, one thing to keep in mind on transfer speeds is also the medium that data is being transferred from. For instance, a mechanical HD will send information slower than an SSD or a flash card because of their inherit speeds already.