We request 3 VMDKs for data (as a minimum) because VMware's ability to create 4 virtual disk controllers per VM. These 4 virtual disk controllers can than be assigned the 4 VMDKs allocated to the Delphix Engine (1 VMDK storing Delphix software, 3 data VMDKS). We have found that that configuration provides maximum throughput regardless of the number of physical disk controllers in use by the VM host.
Additional information on disk configuration can be found here.
Virtual Machine Requirement for VMware