Question about network interfaces - BIG-IP VE 12.1.2

Hi everyone. I'm new to F5 devices and trying to build up an environment to provide LB to CA API Management Gateways. I'm running the latest BIG-IP VE 12.1.2.0.0.249 virtual machine on VMWare ESXi 6.0. I tried to configure either HTTP and HTTPS but it's still not working. My question is about the interfaces. I noticed that there are 4 NICs provided by default during the installation: management, internal, external and HA. I know that the HA is not mandatory. But what about the other 3 interfaces (mgmt, internal and external)? Do I need to have 3 separated subnets?

Right now I only have my internal subnet, where the mgmt is actually running, and a VMWare subnet, where I put both the internal and external.

Thank you for any help.