This is one of the standard ways of indicating the wind speed in a given location.

"Basic Wind Speed (V)—The 3-s gust wind speed at 10 m (33 ft) above the ground associated wtih a 50-yr mean recurrence interval [...]
Three-Second Gust Wind Speed—The average wind speed measured over an interval of 3 s."
Guide Design Specification for Bridge Temporary Works
http://books.google.es/books?id=acFaw4R2jdkC&pg=SA3-PA1&lpg=...

"How to Convert a Three-Second Gust to Basic Wind Speed.
Wind is measured in both sustained wind speeds, and 3-second gusts. When you see a weather report that gives "Winds out of the southwest at 25 mph with gusts of up to 40 mph", the 25 mph is the sustained wind speed, and the 40 mph velocity is a measured 3-second gust."
http://www.ehow.com/how_7407867_convert-gust-basic-wind-spee...

So the wind speed averaged over a period of three seconds is taken as the standard definition of the gust speed, and "a three-second gust wind speed of up to 52 m/sec (115 mph)" means that 52 m/sec or 115 mph is the highest average speed measured over a three-second interval.

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Note added at 1 hr (2014-05-01 15:01:13 GMT)
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The point is that wind speed is constantly fluctuating, and any speed will be the average speed over certain period of time. For gust speed, three seconds is that interval. But if the gust speed is up to a certain figure, that doesn't mean that the figure is the maximum speed reached for a given period, but the average speed over that period.

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Note added at 1 hr (2014-05-01 15:19:01 GMT)
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Sustained wind speed is average speed over an inverval of one minute, and peak gust speed (normally higher than sustained speed by a factor of about 1.22) is the highest average speed over a consecutive period of three seconds within that minute. These figures are used to classify hurricanes, for example.