The Nokia displays are already "serial" in the sense that they use SPI. This also means that you can use the built-in SPI library. Tie the "chip select" of each display to a different digital pin, but have them share SCK, MOSI and MISO. When you want to talk to a particular display, activate its chip select pin, and use MOSI to tell it what to do.
Yeah that's the gist of it, but the PCD8544 library appears to store a buffer for what's displayed on the LCD so if you had only one instance initialized all three screens would share that buffer.
It looks like the straightforward method would be something like:
// pin 7 - Serial clock out (SCLK)
// pin 6 - Serial data out (DIN)
// pin 5 - Data/Command select (D/C)
// pin 4, 8, or 9 - LCD chip select (CS)
// pin 3 - LCD reset (RST)
PCD8544 nokia1 = PCD8544(7, 6, 5, 4, 3);
PCD8544 nokia2 = PCD8544(7, 6, 5, 8, 3);
PCD8544 nokia3 = PCD8544(7, 6, 5, 9, 3);
...then normal stuff.