You can set this sort of “redundancy with different-size drives without wasting a bunch of space” thing up at the block device level – I understand that Synology’s “Hybrid RAID” is a Linux system doing that. But you’ve got to be careful doing that; configure it wrong and you won’t have redundancy.

I don’t know, somewhat-surprisingly, of a software package that aims to manage a collection of disks to do this configuration at a high level, slicing up a collection of drives into smaller block devices and adding and removing disks and migrating data and such while providing guarantees that data has at least N drive redundancy.

That being said, even in such a configuration, you can only do so much. If you have one drive that’s 10TB and the rest of all your drives intended for redundant storage sum to 3TB – which is what you have – you can’t have a configuration that can handle failure of your 10TB drive and can store more than 3TB of data with redundancy, no matter how you slice and dice things. You’re going to have to waste 7TB of space or store data without redundancy.

What’s best to do is probably going to depend on how much you want to spend.