How can I construct a good thesis statement for a history essay about Boudicca?
The first thing you need to think about when creating a thesis statement is what you want to say with your essay. Is there main central point you're making about Boudicca? What will your final statements say about her? You could make a central point about her personality, what she teaches us today, or anything similar. Your analysis could be a good source for this final main point you want to make. This will be the "discussion point" of your thesis statement.
Your thesis also needs to include the three focus points of your paragraphs. As you mention, these are who she was, her revolt, and your analysis. So, you need some keywords or phrases that summarize what you will say in these paragraphs.
Let's look at all of this together. Your thesis statement will something about Boudicca, as in "Boudicca was a British queen who..." and then add your discussion point and three points that summarize the three paragraphs you'll be writing.
A good idea is to write your three body paragraphs before you construct your thesis. These will then give you material that you can focus into a good thesis statement.
I hope these ideas help you to create a great thesis statement.