As Bill Clinton use to say "I feel your pain".
Note leewcraft's post, if your third drive is still available when you boot up your system again, you might be able to finish the re-build before it totally fails. I've never had two drives fail at the same time with my SATA RAID6 system (Areca card), so I don't know if the re-build time is longer if you replace both or just one. The problem with these large size HDs, it takes much longer to re-build if you have 10TB+ in one RAID set, which makes you vulnerable to another failure and loss of the RAID6 when the third drive fails.
Having SCSI drive RAID5 systems with 250GB drives in old days seems like a blessing, also had a lower hard drive failure rate. I won't be going above 12TB in a RAID6, once I found that re-building took 2 days.