hi prevravanth,
If you have 3 external MCU's RS232 output lines connected together 'somehow' , how are you going to prevent more than one MCU using the serial line at any one time?.
Unless the MCU's have some sort of interrupt priority control you are going to have arbitration problems at the PC end. That is, while one MCU is transmitting, the other two MCU's may start transmitting.
The easiest solution I can suggest, is that you install a dual uart pcb in the PC.
Use the existing Com1 for say MCU #1 and the Com3 and Com4 for the other two MCU's.
The program in the PC then can determine which MCU is sending.
If the PC program is written to respond to OnComm events for Com 1,3 and 4 it will not miss any incoming RS232 data
from any of the MCU's.
An additional advantage is that the PC can transmit to each individual MCU.
Eric