Since the 3rd key is useless on its own I don't think it warrants its own paid storage in the form of an extra residence or deposit box, particularly if you're still concerned with ensuring your own physical access.

Could keep the key in a vehicle or geocached underground elsewhere on the property so its separated from the structure. (special archival grade dvd-r or steel product)

Could also be encrypted and stored in-cloud, you already have to remember where these things are are and how to use them so an encryption phrase is lateral lift.

Could also replace it with a brain-key in lieu of encrypting, a phrase you can hash to make a seed can obviate the concern of storage corruption.

Or all of the above for redundancy.

The advantage of 2:3 over 2:2 is the 3rd key can be differentiated paranoia instead of duplicated.

There's also companies like Iron Mountain etc that do physical archive storage, similar trade-offs to the safe deposit box but may offer preferable obscurity and potentially more pseudoanonimity via using a company name.