From the snippet you provided, we don't know what styles are associated with the CSS classes "MsoNormal1" and "sgc-3". You will have to attach the epub file itself.
But you really shouldn't be using <p> tags for chapter headings. Use heading tags (<h#>, where # ∈ [1,6]). There's a WYSIWG action for that, it's the <Select heading> dropdown box. Just select some text and then select a heading level.