This sounds like it will a fairly easy assignment since it is about a topic directly related to you. You can choose anything that has happened in your life that you remember a lot of detail about.
I would include in the first paragraph specific details about the event. Include the five W's: who, what, when, where, and why. Describe everyone that was involved, what happened, when and where it happened, and why it happened.
The second paragraph you can describe how the event made you feel. You can also include how it made others feel.
In the third paragraph you can write about any effects the event had on you or those who were involved in the event. You can also include how things would be different if the event never happened or happened differently (or you reacted differently).
If this is your topic, then there are many things you can write about. First, you would need to decide on a memorable event. You might want to think about the event and ask from whose perspective it is memorable - your perspective, your parent's, the country's, etc. For example, you can write about your first vacation, which might be memorable to you, or the ending of WWII or 9/11. Second, you need to write about why the event is memorable. The better your reasons are, the better your paper will be. I think most of your paper should concern the reasons. Finally, you want to conclude your essage in the last few lines.
I think that an honest response to this question would indicate that few, if anyone, can really generate this except for you. In the end, I think you have to develop a memory of some event which bore importance or relevancy to you. I think that spending some time recreating it for the reader could be a step taken to writing about it. There should be a recreation of literal sensation and containing sensory imagery. There might be some time devoted to its meaning and how it is important to you and what its lasting legacy to you. This is going to have to be developed by you because of the importance to you. In spending some time reflecting and ruminating upon it, you might be able to develop a better understanding of how to write it for an audience.
Well, we would need some more parameters for the assignment to give you some real help on this one. But since you have only three paragraphs, I'm going to assume it's supposed to be about an event that's memorable to you.
I would spend paragraph one describing the event itself, paragraph two telling the reader why that particular event is memorable to you. (Or more memorable than others) The last paragraph I would spend writing about how the event has changed, you, the way you look at the world or one aspect of it, or what you learned from this event.