I don't think having 3 encrypted backups of my wallet.dat will ever fail. In order for this to happen, all my backups need to be compromised (destroyed actually), at the same time, or at a small period of time in which I will fail to realize it and take precautions. It's extremely unlikely that 3 digital media will die at the same time and it's also unlikely that 3 locations will be compromised at the same time.
Apart from forgetting the encryption password, here's another scenario. First: when you say 3 backups, does that include the wallet you're using? I assume so. So by the time your system fails and you realize you need your backup, you'll only have 2 backups left. You're nervous, not very careful, and before you know it, you have only 1 backup left. Or you messed up one of your backups while you created it.
Do you at least check if you can restore from your backups once in a while?