Receive serial data on multiple pins ATTINY85
You could do this with only software. 

You would set up a timer interrupt to fire at least 4x the incoming bit rate. 

On each fire, you sample all the input pins and do some edge detection and bit shifting into input buffers for each of the inputs. 

You could then use the single hardware UART to send the data out, which is nice because you will want the output to be at least 3x faster than the inputs so it does not get overrun when combining the inputs into a single stream. 

This is not too hard to code, but you do have to understand how software USARTs work.