(1) Sample size = 100÷2=500, then the frequency of the third group = 500-50-100-200-25 =125,

(2) Because the median is the average of the sum of the 250th and 25th1data from small to large,

And because the frequencies of the first three groups are 50, 100 and 125 respectively.

Therefore, the median of the sampled samples belongs to the third group;

(3) Because 10000× =500,

Therefore, it is estimated that 500 students in the third grade of the county won the prize.

The total number of (1) is 100÷0.2=500, so the frequency of the third group is 125, so draw a picture casually;

(2) According to the calculation method of the median, the median falls into the third group;

(3) Estimate the population with samples.