Slovenian Primoz Roglic kept the overall leader's pink jersey after the stage was neutralised with nine km remaining due to the treacherous conditions, freeing the sprinters to race for the line while the main men could stay safe in the pack. Colombian Fernando Gaviria, for Team UAE, finished second behind the Bora-Hansgrohe rider with Frenchman Arnaud Demare third for Groupama-FDJ in a stage that was also neutralised for the first 2.7km.
Ackermann had also won Sunday's second stage. "All the stage was scary, all the descents, you cannot see that much in the peloton because of all the water," said Ackermann, wearing the cyclamen jersey for the sprinter classification leader.
"It just was lucky that nobody crashed." The German said it had effectively been a double sprint to the finish for him after he had to brake in the last 250 metres before harnessing all the power in his legs in a final dash for the line.
Injured 2017 champion Tom Dumoulin abandoned the race shortly after the start after a crash on Tuesday, with his focus now on the Tour de France. (Reporting by Alan Baldwin in London, editing by Toby Davis)
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