What you are trying to minimize here is the error rate of the composite system, not the error rate of the individual modules. You take it as a given that all the teams are doing their human best to eliminate mistakes from their design. The idea of this is to make it likely that the mistakes that remain are different mistakes from those made by the other teams.

Providing errors are independent, it's better to have three subsystems with 99% reliability in a voting arrangement than one system with 99.9% reliability.