I'm not entirely sure why you'd do that, but definitely I2C is an option. Most mirocontrollers nowadays have plenty of I2C libraries and peripheral implementation documentation in their resources.
Another thing you could try doing is multiplexing, so you choose only one of the 3 lines as a valid one for whatever you're communicating to.
In any case, I understand that you didn't design that but that you're only studying it right? Have in mind that communication can follow standard protocols (I2C, RS232, SPI, etc...) or you can just make your own so it all depends on how the controllers are programmed. For instance, if the three TxD are all interconnected (properly pulled up or down, and connected through buffers or resistors) but thanks to the programming only ONE can actively pull the line's voltage Up or Down, then it would work.
Try studying the data-flow a bit more, like what should be happening inside those processors and how should they be "talking" to each other, and then we can help out more