A good thesis comes from productive brainstorming and pre-writing. It is always easiest to start an essay by turning the prompt into an open-ended question and listing as many answers and details as possible. An example to help you get started might be:
What three jobs are you most interested in persuing in the future, and why?
Now, make a list. Make a list of more than three jobs if you want, because you might find that in the end, your ideas flow more freely with one than another.
Once you've listed your jobs, list reasons why they interest you. Note: it is very likely that three careers you might be interested in are all similar to each other, or even in one related field. This is not only natural, but probably common. This means for your essay you need to think about how to distinguish them. This can be done by making direct comparative statements from one career to another. For example, if you are interested in the medical field, perhaps you've thought, "I could be a doctor or a nurse." Think about reasons you'd choose one over another. Be as honest as you like. In this example it would be perfectly logical to say that you might choose nursing over becoming a medical doctor because the schooling is much shorter and therefore cheaper.
Once you've brainstormed your list, choose the best three careers for your essay (the three with the most detailed examples of why you like them). These will become your three body paragraphs. In order write your thesis, you simply need to answer the prompt question (from above) in a complete sentence or two and include your three category choices. An example might be something like:
I am interested in the medical field because I wish to help people who are sick or hurt. As a result, my top three career choices are [choice 1], [choice 2], [choice 3].
It is okay if your career choices have absolute no relation to one another. It simply means your essay will have three very distinct body paragraphs. If this is the case, when writing your rough draft, I encourage you to transition between each paragraph with a logical segue into each new idea.