I know I'm no expert in this area but that seems high to me. Your array has only had three drives at a time and you've had about one failure a year? I have four WD drives in my Synology, and they were getting written to 24/7 with the Surveillance Station running for three years, and the NAS has been operational for 4-5 years. So far I've never lost a drive (but now I'm going to kick myself for saying that).

Naturally, YMMV. I would say it could be environmental factors, but my Synology now lives in our laundry room where it's regularly 80 degrees (F). It's been humming away like that for two years now.

...I'd better order a drive to have on hand just in case...