Just to clarify, You want to install FreeBSD on a hard disk and have 3 USB Mirrors??
This is possible, but the problem is the slowest device is the fastest speed you will see.
You could use 8GB USB for a FreeBSD Install and use 2 of your 16GB USB as mirrors.(And have an extra 8GB on each device)
Please elaborate on what you want. All USB medium or USB and and hard disk?