Hi Robert,
For Two-way mirror and Three-way mirror, Two-way mirror is equivalent to having one data copy, while Three-way mirror has two data copies.
The biggest difference between Two-way and Three-way is: Three-way mirror allows two disks to fail, but Two-way mirror only allows one disk to fail.
Whether it is Two-way or Three-way, both have the effect of high availability, effectively perform failover, and ensure the security of data.
For more information, please refer to the following links:
11382.storage-spaces-frequently-asked-questions-faq.aspx
Hope my answer can help you!
Best Regards,
Xu Gu