For example, my understanding is that the redundancy approach is basically "do everything on 3 computers, vote on the outcome".

Sometimes you even have different firmware/software teams write code for each of the three different configurations (often targeting different processors/architectures, etc). Pretty cool

You can "skimp" on hardening if you use more redundancy (which is generally cheaper, and often lighter (which is the real cost savings)).

You can also skimp on hardening if the operational lifetime in space is minutes to days, instead of months to years like most satellites.