It couldn't be easier to use an E3D volcano with your Prusa MK3 - all you need is one part. This part. It is a replacement extruder cover, no other hardware modifications are required. You can even get away with nothing but a volcano heater block and nozzle - the heater cartridge and thermistor cartridge used in the E3Dv6 hotend can be reused in place if you wish.

The volcano heater block will screw onto the existing heat break without issue. Simply transplant the square nut and hex nut used in the stock extruder cover into this new one, screw it on and reattach the print cooling fan and nozzle, adjust the PINDA probe so it is about the thin part of a ziptie higher than the nozzle tip, and that's all she wrote!

Well, you will need to recompile the firmware. It's actually quite easy, you can compile and upload it to your printer all with one click using the Arduino IDE. Instructions and download links for the firmware source code and the right version of the Arduino IDE are available on the offiical Prusa3D github page.

You need to change one line in the file which, if you follow the instructions, you'll have copied from the variants folder and named Configuration_prusa.h.

Find the line towards the beginning of the file (right now it is line 64) that says
#define Z_MAX_POS 210

Change the number 210 to 202, so the line now reads
#define Z_MAX_POS 202

This tells your printer to accommodate the extra 8.5 mm of height the volcano hotend has. Save the changes, compile, flash the firmware, and you're all set!

You will probably want to go to Calibration->Calibrate Z Axis to make sure your height is dialed in correctly, and re-run Calibration->First Layer Calibration. Once those two final bits are complete, you can resume using your Prusa MK3 just as you had been before.

Final note: If you can't seem to be just squirting plastic at the much higher rates you were expecting and you use Slic3r as your slicer, it is probably because of the volumetric limit for a given filament. Go to the filament settings tab, select the Advanced section, and find the setting labeled 'Max volumetric speed.' It defaults to a setting that makes sense for an E3Dv6, but not your E3D Volcano. Take whatever number is there, triple (yes, triple) it, and that is a good starting point for the Volcano. You will also need to increase your extrusion width, layer height, or print speed, or any combination thereof, to really benefit from the plastic riot hose you've installed.

I find the 0.4mm volcano nozzle works well for me - you can use extrusion widths as wide as 1mm even with a 0.4mm nozzle without issue.

Enjoy!