I assume that what you want is the three sides to all print as one sheet and that the book is of a size where this can be done.
You could create a document in landscape orientation and insert a three-column table with no borders. Size the three columns to be your covers and spine.
Then insert your graphics. You might do best saving your PowerPoint slides as high-resolution pictures and inserting them into the Word file. Make sure you tell Word to not reduce the resolution.
This is not something I have done, I am describing how I would go about it. Publisher might be a better pick than Word.
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