Three laps? Come on, PRO! Give our grey cells a break.
Too bad. Sailed extra distance. No rule against that. 
Score as finished.

I would say that they finished when they crossed the finishing line. There is no rule against sailing extra distance.
They met the definition of sailing the course.

The Viper 640 and multi-hull classes at my Club often continue to sail if there is wind and daylight on our Wednesday night series. The boat that brings in the start/finishing mark gets a beverage(s) of choice on the race committee.

When the first two boats (Melges 24s) crossed the finishing line after sailing 6 legs, they did not still finish. Because they continued to sail the courses under the definition Finish (c).

When the two boats crossed the finishing line after sailing 8 legs and then sailed home, they finished correctly.  The race committee shall score two boats based on their second crossing of the finishing line.
Their scores might be significantly worse, but it  is due to their own voluntary actions.
(Ref: US Sailing Appeals Book: Question 86)  

I thought you have to cross the line from the course side.  Is that not the case?

To clarify, the two boats only "finished" once, after the seventh rounding.  Sen Yamaoka, thank you for your Appeals citation.

I think it's important to know if the "finish line" and it's marks are separate from the "leeward mark" or equivalently when they did the final downwind rounding, did the also cross the finish line from the course side.   

There is a funny "chicken and egg" problem with the definitions of Finish and Sail the Course.  You haven't Finished if you continue to Sail to Course, but you don't continue to Sail the Course once you've Finished.  What to do???

Assuming they actually crossed the finish line at their last rounding I would say that they have Finished.  Their "string" will have satisfied the course requirements in Sailing the Course as the other legs and rounding they did are not part of the course.  A RC, at the instance their bow crossed the line, would mark them as finished, and who would know what or why they were taking their subsequent actions.

Chrostopher Walmsley - The finish line was separate from the leeward mark.  I've copied a (not to scale) drawing below.  They did not cross the finish line after rounding five marks (as a W6 would require) but went on to round a sixth (leeward) and seventh (windward) mark and then finished between the RC and pin.

Robert ... the language is quite clear I think.   They sailed the race (RRS 28) and in doing so included a valid start and finish and they sailed the course.

A3. STARTING TIMES AND FINISHING PLACES

The time of a boat’s starting signal shall be her starting time, and the order in which boats finish a race shall determine their finishing places. [...]

Finish

A boat finishes when, after starting, any part of her hull crosses the finishing line from the course side. [...]

28. SAILING THE RACE

   

A boat shall start, sailing the course and then finish. While doing so, she may leave on either side a mark that does not begin, bound or end the leg she is sailing.

A boat sails the course provided that a string representing her track from the time she begins to approach the starting line from its prestart side to start until she finishes, when drawn taut,

I agree with Angelo, that they still sailed the course.  The fact that their "string" went around some unnecessary marks shouldn't matter if the passed the necessary marks in the correct order.