This page from mathsisfun.com provides several examples showing how to calculate the mean of a set of numbers. One of the examples involves negative numbers. At the end, you can link to "the mean machine" or take a quiz. For a more advanced discussion about mean, see http://www.onlinemathlearning.com/mean.html.
Read about the median at mathsisfun.com. At the bottom of this page, you can try finding the median of different data sets, and you can take a quiz on medians. For a more advanced discussion of median, see www.onlinemathlearning.com/median.html.
This site from mathgoodies.com provides several examples showing how to find the mode for a set of data. At the end, you can try a quiz. For a more advanced discussion about modes, see www.onlinemathlearning.com/mode.html.