If 3 was listed as a measure of central tendency, then the mode was used.
The mode is the value that appears most often in the data; this is 3.
The median is the middle value after the data has been ordered from least to greatest. From least to greatest the data is
2,3,3,4,5,6,7,8
Since there are 8 data values, the median is between 4 and 5:
(4+5)/2 = 9/2 = 4.5
The mean is found by adding the data values and dividing by the number of data values:
(2+3+3+4+5+6+7+8)/8 = 38/8 = 4.75
The range is the difference between the least and greatest:
8-2 = 6
The only one of these that is 3 is the mode.