a mod to receive 3mm ?

Lemme guess... 3Doodler?

Sure can.

Need to replace the extruder cold-end setup and then replace the hotend with a 3mm one. Basically you need to redo the entire extruder sub-assembly and hotend.

You can fabricate the extruder yourself - keeping in mind that the alignment will all need to change to accomodate the nearly doubling of width of filament... oh and you'll probably want to dial up the vref a heap, which could be an issue on the stock motor (you need more torque to move 3mm than 1.75mm...) . But hey - it might work with the OEM one. Alternatively, throw the whole thing out and buy a Bulldog XL (must be XL to support 3mm).

Hotend wise - you can buy a 3mm whatever. J-Head/E3D/Hexagon - in 3mm you can even use Magma hotends and the Prusa nozzles...

Separate to all this though is that 3mm filament and printing is different to 1.75mm printing - things like filament tension, motor torque, pressure, temp and speed all become even more critical....