MCU choice, 3Mbps UART,
Take a look at the Atmel SAM3S.  It is not a 16 bit device but I'm not sure why that is a key requirement.  I'm also not sure what "low" power means to you.  This will be in direct conflict with the requirement for the fast baud rate.  To hit the 3.25 MBps you have to run this part at 52 MHz (It runs at 64 MHz max).  Another nice feature it has is DMA to help move the high speed serial data.  It also meets your memory requirements.