Degraded Array. RAID 6 with three disk failure

Solution 1:

As said before, if more than two disks in a RAID-6 array die, the array is unrecoverable.

However, three simultaneous disk failures are quite an unlikely event: it might very well be a case of a faulty enclosure, backplane and/or controller.

You should try removing and re-inserting the disks, replacing the controller and/or the enclosure, and even putting the disks in a different server with the same controller (if you have one available).

Solution 2:

You don't give any details on the server type, RAID controller type or anything specific.

Try turning everything off for 10 minutes... Remove power from the server. Let the drives spin down.

Power the server back on and see if the RAID controller re-recognizes the drives and is able to boot.