If you are using a Layer 3 switch with separate VLANs, wouldn't the switch have a defined IP address for the Default Gateway defined for each VLAN (typically 192.168.xxx.254 )? Then you would put that into the IP4 configuration of the HMI PC?
So let's say you connect the HMI to VLAN 1. The IP Address could be 192.168.1.200 and the Default Gateway 192.168.1.254 , then it should be able to connect with all 3 PLCs