I need to comunicate serial with 3 devices:
Assuming those devices talk back to the mcu (they are write-only devices), the trick is to use a resistor + mcu pin to isolate them from the bus: from the serial bus, run a resistor (1k) to the device. On the device's serial pin, connect a small resistor (110ohm or less) to a mcu pin (UART_ENx pin). Default the UART_ENx pin to set / output.
When you want to talk to this device, tri-state its UART_ENx pin; at the end of the transmission, turn it as output.
This approach allows you to connect as many write-only uart devices to one hardware uart.