Hi,
Here is how you can estimate the total usable space in the storage of your HCI
I understood that, there are 3 disks total with total capacity of 69 TB available in the cluster
Leftover space of 7 TB will be divided in to 3 and thus 2.x TB is available as usable space
Future Growth is a reserved expected growth. This is also available for immediate use. However, any future storage plan with this future expected growth will be impacted