If you use a MAX232 driver for all three 8051 RS232C interfaces it is possible to parallel the three txd outputs by connecting a switching diode's anode to pin 7 of each of the MAX232's. To the cathode connect a 47k ohm resistor to pin 6 of the MAX232. The output (cathode) is then pulled to the mark (-10 V) condition. This allows several MAX232 drivers to coexist with similarly configured MAX232's. Each slave will require a unique Identification number that is to be polled by the master. This can be provided when you program the flash or EPROM or by perhaps a dip switch installed on your printed circuit board.
OUCH, making something simple complicated and non-standard.
Probably the easiest thing to do is use a p89c668 or similar no need, IIC can be implemented on any uC and use the I2C interface protocol. This will save using the MAX232's.
IIC is probably the best if all 3 controllers are on the same board and you have the time (100/400 kbits/s). If you need a better performance use DualPortMemory.
If the controllers are not on the same board I would use RS-485.

Erik