1-Wire Serial communication to standard RX/TX Serial ports in microcontrollers
With some software effort you can read all three sensors over the same wire: 1-wire peripherals have a unique ID. 
Dallas 1-wire is a specific communication protocol, and using UART hardware to communicate with a 1-wire device is, well, like using a chainsaw to turn a screw? Better look for a 1-wire library that simply bit-bangs the protocol. There are bound to be many such libs, either for the Arduino or for a bare Mega 2560. 
If you really really want to 'split' the one (UART) line you could use an analog multiplexer.