I can't think of any pros.
The cons are:
you're trying to make a RAID with disks on buses of two vastly different speeds
you're trying to make a RAID with a bus that has notoriously poor low-level drive discipline options (USB)
given the bandwidth limits of the USB bus, you would risk swamping the controller (particularly given that you want your spare on there) and corrupting the array regularly
you're introducing a "spare disk" to a RAID 1 setup, which doesn't really make much sense (they're both already "spare" in a RAID 1)
Basically, you're taking one fast disk and two slow disks, slowing them all down to the speed of the slow disk, and limiting yourself to the size of the smallest disk, while introducing new points of failure, and getting no benefit from it that I can see. Just use the USB disks as backups.