One option would be a Triscend E502 device.
http://www.triscend.com/products/indexe5.html

One of the ports would use the dedicated 8051 UART. The other two ports would be implemented using the UART modules from the Triscend FastChip library in the E5's Configurable System Logic (CSL).

The E502 also has 8K-bytes of on-chip XDATA RAM that can be shared amoung the ports. You could create separate read and write buffers. The master would fill or empty the buffers for the two slaves.

You would still have additional on-chip programmable logic available for other functions, as well as lots of PIO pins.

There are three larger devices in the E5 family if you need additional memory or more programmable logic, or more PIO pins.