AVR-Chat

Im working in a project with atemega8 and atemega32, for this work is 
necessary 3 serial ports, and I cant loose any data, but what happen if 
two or more ports have incoming data at the same time?, what can I do?, 
other question is what is the adventage for use usart hardware or usart 
software?.

Sounds like a good case for using interrupts.
Software uarts require a fair bit of software overhead to run, but if you
need three uarts, you're pretty much stuck with that.

OTOH, I seem to remember a data sheet for an AVR with four hardware uarts.