we have four different PLCs with the same IP addresses, and we need to communicate these four PLCs to our IoT gateway. Since PLC has the same IPs we can’t communicate to our device. And for that, we plan to add the L3 Switch in between PLCs and our IoT gateway.

Your topology is not workable，because different PLCs are with same IP address.
There are two options：
1、change IP of PLCs.different PLCs are with different IP address.
2、Add NAT equipment between PLC and switch。in other words，the PLCs seen on the switch are different IPs.