If you rounded to the nearest whole number it'd be 2345, which obviously has four significant figures. So you need to round 2 344.682 8 to the nearest decade.
By the way, a rounded answer of 234 does not seem very close to 2345 .... (common sense should always be used to check an answer).
Alternatively: In scientific notation your number can be written 2.3446828 x 10^3. If you only want three significant figures, it becomes 2.34 x 10^3 which can obviously be written as 2340.