how to use uart 3 in lpc1768???
What does the lpc1768 documentation tell you?
www.nxp.com/.../lpc1700-cortex-m3:MC_1403790745385
How does it differ from the other UARTs?
Note that this has nothing to do with Keil; it's all down to the NXP hardware - which neither knows nor cares what software tools you use.
http://community.nxp.com/
www.youtube.com/.../LPCZone
www.nxp.com/.../training-events:TRAINING-EVENTS?tid=FSH#/