One usually successful introduction for several types of essays, including a cause and effect essay, is to open the introductory paragraph with a series of statistics related to your topic. In this case, your topic is homelessness, and there are many statistical analyses of this problem in the US--for example, you can find the overall number of homeless people in the US; a breakdown of homelessness by state, county, age, race, and several other components of homelessness. After you have done some research in order to understand the scale of the problem, you can choose which statistics support your overall goal in the essay. Your introduction, then, will begin with a discussion of statistics, facts that no one can dispute. The second part of your introduction will contain a description of the causes and effects. Be aware, however, that if you have three causes, you need three effects (unless two of those causes are so closely related as to be one cause). As you research homelessness, among the statistics you will find several of the most common causes and, most likely, the a discussion of effects. Your research will probably lead you to more causes and effects than you can reasonably handle in the time and space you have, so your task will be to pick the most important two or three out of the many that you will find.
In the body of the essay, you will discuss the two or three causes and effects that seem most important to you. I suggest you deal with one cause and its effect in a paragraph; the second cause in another paragraph, and so on. In your discussion, you should pull relevant statistics into your discussion that apply to that particular cause and effect, and these statistics may already be in your introductory paragraph or may be other statistics that you haven't mentioned yet. As I noted above, one of your primary goals is to write a balanced essay so that if you write about a cause, you follow with an effect: three causes; three effects.
Your concluding paragraph is often the shortest. Here, you simply try to sum up your most important statement about the causes and effects you discussed in the body of the essay. Often, you will have decided, based on your research, which cause and effect is the most important of the three you discussed, and the conclusion will summarize the importance of that particular cause and effect. Cause and effect essays are not necessarily argument essays, but it is quite natural to conclude that one cause and effect is more important than all others, and you leave the ready with that statement.