Ah, interesting challenge. The thing that makes this tough is how short it has to be. There's tons that could be said about these plays, so you want and need to find a single focus. Since these history plays all deal with political power, I'd probably go there. What can you say about power in these plays? Look at them to see what they have in common; can you, for example, write about the cost of power in each play? About where the source of political power lies, or the right to rule comes from? Or you could always try the moral route: what do these plays teach us about how power should be exercised? Find one or two of these common threads, and pull the most important example from each play.