From an 80C51 family Philips manual:
"Mode 3 is provided for applications requiring an extra 8-bit timer on the counter. With Timer 0 in mode 3, an 8051 can look like it has three Timer/Counters."

When timer 0 is configured in mode 3, timer 1 looses some functionality, but it can still be used for baud rate generation.

Maybe this helps.

Alfredo del Rio.