To port forward traffic to a specific port on specific VMs use an inbound network address translation (NAT) rule.
https://docs.microsoft.com/en-us/azure/load-balancer/load-balancer-overview an inbound NAT rule :
Create a load balancer inbound network address translation (NAT) rule to forward traffic from a specific port of the front-end IP address to a specific port of a back-end VM.
Hence this option is Correct
a load balancing rule : Incorrect Choice
A load balancer rule defines how traffic is distributed to the VMs. The rule defines the front-end IP configuration for incoming traffic, the back-end IP pool to receive the traffic, and the required source and destination ports.
a new public load balancer for VM3 : Incorrect Choice
This option will not help you since this will route all traffic to VM3 only.
a frontend IP configuration : Incorrect Choice
When you define an Azure Load Balancer, a frontend and a backend pool configuration are connected with rules. The health probe referenced by the rule is used to determine how new flows are sent to a node in the backend pool. The frontend (aka VIP) is defined by a 3-tuple comprised of an IP address (public or internal), a transport protocol (UDP or TCP), and a port number from the load balancing rule. The backend pool is a collection of Virtual Machine IP configurations (part of the NIC resource) which reference the Load Balancer backend pool.
Reference:
https://docs.microsoft.com/en-us/azure/load-balancer/tutorial-load-balancer-port-forwarding-portal
https://pixelrobots.co.uk/2017/08/azure-load-balancer-for-rds/