The "multi-prong" plugs are a carryover from G/A and rotary engines - General Aviation uses them because it spreads out the HV erosion, and allows for longer effective service life. Rotary engines use them to reduce plug protrusion and keep the rotor apices from smacking the plugs.
(In some cases, a replacement plug has been devised that simply uses a ground "collar" around the centre electrode, more common in G/A than in automotive applications. I'd like to see those come into the automotive applications more, tho - no need to check/set/correct the gap, and HV erosion is spread around the entire collar.)
ghoulardo (interesting choice of moniker) is entirely correct that there will only be one spark at a time, and it will only jump over one gap. There are some sound engineering reasons for having multiple gaps available, but the necessity of such is often debatable.
(If this is an old post, my apologies. I just found this board, and found this post looking for the ID of the spark plug wells - so I can figure out what I need for an OD on the spark plug socket...)