Failing Disks - is it me?

[racingtortoise]

First, I dont fully understand the disk technologies and settings, so hope what's below makes sense to someone.
I have an AS-604T (purchased 2013) populated with 4 x 3TB WD Red disks and configured as RAID 5.
I've had 3 failures (different disks), each with the S.M.A.R.T report indicating "Current_Pending_Sector Bad".
Removing a failed disk and testing with WD's stand alone diagnostic shows no errors.
Reinstalling the disk, synchronising, and then scanning with Disk Doctor shows no bad blocks but the S.M.A.R.T error persists.
I find it odd that three of the disks should have had this issue & I've managed to get one replaced under warranty, but asking for more would surely create suspicion.
1. Does the failure I've described mean each disk should be replaced?
2. With three failures, is that "normal" or just bad luck?
Any other guidance / suggestions appreciated (including trying an alternate make).

[orion]

SMART values are statistic data. It means that the disk might be going to fail in the near future. If I were you, I won't change disk at this stage until I see RAID-5 volume is degraded (one disk failed).

[racingtortoise]

Thanks. I'm just disappointed at getting three failures on what I considered god quality disks.