Do the three iPad personal hotspot tethering options differ in terms of speed?
USB is probably the fastest, with a theoretical peak transfer rate of 480 Mbit/s, and will also charge the device (or, at least, prevent it from discharging as quickly). The downside is that only one device can be connected via this method.

Bluetooth is probably the slowest (Bluetooth v3.0 + HS is rated for 24 Mbit/s), but I'd expect it to use less power than WiFi tethering. Bluetooth is also limited to connecting to one device at a time.

WiFi tethering will support more than one device (usually three or five or more) and can be faster than Bluetooth, but uses more power. 802.11g is rated for 54 Mbit/s and 802.11n is supposed to reach 150 Mbit/s, but this is usually shared between all connected devices. 

Despite the single-device limit, USB and Bluetooth connections can, if made with a PC or Mac, "forward" the connection over WiFi (as an ad-hoc hotspot), FireWire, Ethernet, or any other connection protocol the machine supports, allowing a theoretically infinite number of devices to use the cellular connection. Of course, you can also use the USB cable to charge the device while using wireless tethering, and you can use multiple (or all three) tethering modes at the same time. 

In any case, the limiting factor for speed is likely to be your cellular connection, particularly if you are limited to 3G. Remember, the cell connection is a single pipe that all your connected devices go through, including the iPad itself.