Just to clarify, vCenter Server as well as the 3 ESXI hosts are running as virtual machines on the hardware ESXI host, i.e. the vCenter Server is not a nested VM in one of the virtual ESXi hosts?
How did you configure the virtual hosts' network settings? Did you assign multiple virtual NICs? Does any other VMkernel port use the same subnet as the Management network? A screen shot of physical ESXi host's vSwitch configuration which shows how the VMs are connected might help.
André