LVM purpose is not to provide redundancy, it only abstract çs the concept of "physical disk". You'll want to use software RAID1 for that.

I suppose that LVM will write to the first disk is the group, then second when its full, and so on. I suppose that you just cannot rely on it though, and assume that if you have 3Tb of data, and 3 disks of 2tb, 2tb and 4tb, the data will only span accross the fiest two ones...