if as I suspect, ascii means ascii coded data over rs232 serial, then I suspect you could find a microcontroller that supports rs232, substract '0' to to your figures comming in, converting them effectively to BCD, and use the precious data to drive 3 7 segments displays. Make that 2 pins for serial in, 2 pins for serial out if needed, 7+3 pins for the display. That should leave you a couple of pins for flashing leds, triggering buzzers, drinving lcds over Mr RB. 1 wire interface or anything else you fancy.
You will need a signal level converter for RS232, a mcu and 3 7 segments displays + 7 resistor + power supply .
I suspect you might even be able to steal power from the serial port, which provides a nice stable high when idling. I am not sure it would be quite enough to drive the seven segment display though.