When you say you want to get
digital data on the "RX" and "read" it every 3 sec, if the data is for example 0011 ...
I wonder if the device at the other end is actually sending serial data in the sense that the USART expects - with a baud rate, start and stop bits etc.
If it is just sending a series of 1s and 0s then you can "read" that on any of the digital pins if you write a program that checks whether the pin is HIGH or LOW at suitable intervals.
Tell us what is sending the data and preferably provide a link to the datasheet for it.
...R