I don't see what the number of extruders has to do with powering your raspberry pi.
Step1: Purchase a decent quality step down DC to DC converter that takes whatever voltage input from your printer's power supplies output (most likely either 12 or 24 volts) and outputs 5v at 2.5A
Step2: Connect the input of said regulator to your printer's 12/24v DC output
Step3: Connect the output of the regulator to the ground and +5v pins on your Pi's GPIO headers Be warned, this can severely damage your pi if the regulator outputs more than 5 volts as you're now bypassing the onboard pi regulator
Step4: Marvel at your success.
While not on a printer, this is how I'm powering my pi3 + screen little mini computery thingy that I made, so while it is "safe" to power the pi straight from the GPIO pins, you need to make sure your regulator isn't outputting any more than 5 volts, and doesn't fluctuate if your printer's power supply voltage output jumps around when the heaters turn on / off.
A safer method could be to take the same sort of regulator that takes 12 or 24 volts input (depending on what your printer's power supply puts out) and spits out 5v from a usb plug, and power the pi in the "normal" way.