On Bantu, validators choose to trust organizations when they build a quorum set. If you are a trustworthy organization, you want your presence on the network to persist even if a node fails or you take it down for maintenance. A trio of validating nodes allows that to happen: other participants can create a quorum slice for your organization that requires ⅔ of your validating nodes to agree. If 1 has issues, no big deal: the other two still vote on your organization’s behalf, so the show goes on. To ensure redundancy, it's also important that those three Full Validators are geographically dispersed: if they're in the same data center, they run the risk of going down at the same time.