1) Purchase short length of 'chocblock' at Homebase or similar. Its a terminal block for joining wires - the smaller one rated at about 5A would be adequate enough for lighting.
2) Switch off power at the mains.
3) Remove old light. Cut off a 3-section piece off the chocblock. Insert the three old wires into each of the three sections - it just isolates them from one another. You just want them separated safely.
4) Wrap insulating tape around the whole piece and push back into wall (if you can - plasterboard studwall only - otherwise it may have to hang behind the unit)
5) Switch power back on at mains.