I am not a ESERIES/StorageGRID guy. But, as I read, it looks like - ESERIES/StorageGRID uses 'DDP' (default and recommended setting). DDP mode provides more efficient recovery from drive failures. 3 Failed drive (as you mentioned) concept is for RAID-TEC and 2 for RAID-6 which I doubt applies to your case?
Operation when a (multiple)drive fails:
A major benefit of DDP technology is that, rather than using dedicated stranded hot spares, the pool itself
contains integrated preservation capacity to provide rebuild locations for potential drive failures. This
feature simplifies management, because you no longer have to plan or manage individual hot spares. It
also greatly improves the time of rebuilds and enhances the performance of the volumes themselves
during a rebuild.
A large pool can continue to maintain multiple sequential failures without data loss until there is no additional preservation capacity to continue the rebuilds.
I don't know how big your POOL is, but ideally it will reconstruct the failed-drive data onto the rest of the DRIVEs in the pool. Only concern is - Overall free capacity will drop. However as long as the POOL has enough free capacity it will rebalance/re-distribute itself with in the remaining drives in the POOL and will continue to serve data. I am assuming once the failed drives are replaced, POOL can be expanded to increase the free capacity. Anyway, hope this helps to reduce some of your worries but you need to keep an eye on the free capacity of the pool.
Useful thread:
https://community.netapp.com/t5/EF-E-Series-SANtricity-and-Related-Plug-ins/Need-to-remove-failed-disk-from-pool/m-p/436667#:~:text=When%20a%20disk%20....
Dynamic Disk Pools:
https://www.netapp.com/pdf.html?item=/media/12421-tr4652.pdf
It would be helpful if you could share details such as DDP POOL size and free capacity etc. so that someone from Storage GRID can help you further (screenshots will be helpful).
