Hi,
I don't think you'll find any AVR or for the matter PIC in a DIP package that has 3 UARTs. 2, I think, is the max. If you use SMD components, then there are some choices. You could use the 2 hardware UARTs and use 1 software UART. mikroC and mikroBASIC (mikroPASCAL as well) provide library routines for software UART, which is very easy to implement. This is likely to be a more cost effective choice than using 2 AVRs and linking them and unless, you're looking at very high data communication speeds, I think this should be fine.
Hope this helps.
Tahmid.