Where is this 3-node minimum mentioned? URL please?

Edit: Looks like maybe you're referring to the recommended 3 node minimum in production mentioned at https://cloud.google.com/spanner/docs/instance-configuration

While I'm not familiar with Spanner's inner workings, I would guess that they recommend 3 instances for quorum establishment in case a region becomes unreachable. If that's the case, using fewer than 3 instances could cause major problems.

Since Spanner currently only supports single-region deployments, it clearly isn't recommended as protection against a region becoming unavailable.

It may be recommended as protection against an availability issue on an instance, though, which is, after all, a big reason why you'd want a distributed DB in production.

I suppose the loss of a region doesn't apply (yet), but yes, the quorum requirement would still apply even if you only had instances in a single region.

Even within a single region you can put three nodes in different zones.