combining RS232 messages

This sounds like a software problem. Call any US help desk to get a talented Indian software engineer on the phone.

Yes They will be transmitting data simultaneously. The computer does not need to send any data. Can any other pin apart from pin 2 of the 9 pin serial port be used for receiving data?

Afraid not. THis might be most easily dealt with by a dedicated controller with 4 serial ports: 3 inputs, one output, being smart enough to keep packets coming from the different inputs separate and passing them on to the main computer. Something pretty smart really.

The simplest thing coming to mind is: let the computer sort it out but give it one serial port for each device. THere are inexpensive USB to serial converters that give you four serial ports for one USB connection.

You need an NMEA multiplexer - there are several links on my GPS/NMEA site listed below, in the NMEA Programs area.

I saw the GPS multiplexer and it is exactly what I need. The problem is I dont think I can get it in India. Is any circuit diagram scematic available whic I can use for makin a simple 2 in 1 out multiplexer?

Will they be transmitting data simultaneously?

Do you need bi-directional links i.e. does the computer need to send data / commands to them?

I think most of the vendors listed on my site will ship naywhere.

It isn't too simple.

What you need is a microcontroller with one serial port for each source, and enough memory to hold at least one sentence from each port. The microcontroller will read and store incoming data then send it out when it gets a full sentence, providing it isn't already sending data from another input.

Or sufficient small microcontrollers, each with one UART and enough memory for a sentence, plus a supervisor (which could be one of them) that acts as a traffic controller. Request-to-send, clear-to-send, and an open collector buffer perhaps.