HP Smart Array P410i, two drives faulty in a short time window

Question

I have a P410i with two disks in RAID 1 configuration.

After some time, a third standby disk was added.

Then, one of the two disks failed, and we ordered a replacement disk. Unfortunately the part was out of stock and supply was delayed. I thought this of little importance, except that a week later, while we were waiting for the disk to arrive "any day now", and well after the array had re-synced, the second disk failed too (I had not removed the failed disk even after syncing finished, and I planned on pulling it out and replacing it in one fell swoop).

Now, the array is running on the third disk. I've placed an order for another two disks, but aren't going to wait for them - the first replacement arrived today.

This is RAID1, not RAID5; I am reasonably confident that nothing has changed and I can pull both failed disks out at the same time, and push the replacements in. The third disk should (after a while) go back to being a hot standby drive.

Are my assumptions correct, or is there something I should be aware of?

Also, the third disk was purchased from the same supplier and from the manufacture date it is two months older than the two failed disks. Should I be planning on purchasing a further replacement, even if it spent all its life in a standby slot and is currently (touch wood) working, just to be sure? I've had disks straight out of the package fail on me inside of a week, so also replacing the disks is not risk-free.