This means that you can lose an individual node and still maintain quorum.
The three node cluster configuration is supported for dev/test because you can safely perform upgrades and survive individual node failures, as long as they don't happen simultaneously.
There are many commonly asked questions about what Service Fabric can do and how it should be used.
There are other issues with Large virtual machine scale sets currently, like the lack of level-4 Load balancing support.For production workloads, you must be resilient to such a simultaneous failure, so five nodes are required. Service Fabric stores state on local, ephemeral disks, meaning that if the virtual machine is moved to a different host, the data does not move with it.