I'll check that one step at a time.

First disconnect everything from P3 and write a simple loop to turn the from o to 0xFF, use a CRO and you should be able to see waveform from all pins. Use a longer delay loop if you want to see it at a lower frequency.

Once it's working, then use a loop to send a char like 0xAA or 0x55 ( do not use interrupt first, just use polling of the TR flag), you should be able to see that with a CRO to check all the timing. (Get the baudrate, parity...etc correct)

Once you get that running, change it to interrupt driven.

Good luck

Lawrence