Communication between 3 Atmega controllers
Hard to say.  If these TXD and RXD pins are really dedicated to RS232 serial, then there would be a multi-driver conflict with the TXD pins.  Also, they would not be able to talk to each other.  

There are a couple of possibilities, though.  I2C, CAN, and half duplex RS-485 are all shared busses that are connected in this way, though the pins are not TX and RX.  In I2C you have clock and data.  In CAN and RS-485, you have a single differential pair for the data.  It's also possible they are using a UART, but they would have to have some method of crossing over TX and RX.  Possibly they are disabling the UART on the transmitter end, turning the RX pin around, and bit-banging it.  Or perhaps the microcontrollers can swap the TX and RX pins.  Or perhaps what you said is not correct and the controller TXD is connected to RXD on the others, and vise-versa, with the TXD pins disabled when not transmitting to avoid conflicts.