Let's say I buy 3 HDD from the same batch to set up a Raid 5. How likely am I going to have 2 drives fail at the same time (I'll have time to do replacement if one of them fails, but not 2)? I am using this RAID setup as the backup (heavy writing but not reading very often)
Seriously, though, I doubt that the distribution of drive lifetimes would have such a narrow peak that a 2nd drive would fail before you had time to swap the first. If you're really worried about that, "burn-in" one of the drives for 48 hours, a second for 24 hours, and the third not at all, and then set up your array.
I have only seen your situation (2 drive failure) happen once, with in a week of each other. And yes the user received the drive the afternoon of the second failure. So all was lost in the 4 disk array. Since you are only using 3 drives you could setup a hot spare. If you are that concern, it is always a good idea to have a spare around. Mfg are always updating firmware. I have seen this in one case where the newer firmware version had less storage space than the original. The raid would not rebuild. Needs to be = or >.
If you are using Maxtor's definately have a spare. I have yet seen one make it through the warranty period. Unless it was a OEM <1 yr warranty.
I have a good recovery services that is cheap, compared to most. Does RAID's too.