CM RS 232 ptp module first 2 LEDs are blinking orange color while modbus communication.

The 3rd Led unlit or not blinking.

What is the function of 3rd led?

Hello,

if you have some questions about used modules, you need check respected module manual/documentation where you find all infos you need.

S7-1500 CM RS232 BA module PDF

This LED is MAINT indication as documentation said. You have there also combinations and flashes for next investigation.