Hi,
You can check the resiliency type to see if it is a two-way or three-way.
100g is only a theoretical size. In fact, the disk metadata and some configuration files also take up part of the space, so this is a normal phenomenon
For more question about this, please check:
https://social.technet.microsoft.com/wiki/contents/articles/11382.storage-spaces-frequently-asked-questions-faq.aspx#What_are_columns_and_how_does_Storage_Spaces_decide_how_many_to_use
Best Regards,
Daniel