A family of Microsoft word processing software products for creating web, email, and print documents.
By default, Ctrl+Q should be associated with the ResetPara command which reverts paragraph formatting to that of the underlying style. On your system, that is apparently not the case.
In the Styles pane (Ctrl+Alt+Shift+S), click the Style Inspector icon
and then click Clear Paragraph Formatting. Does that make a difference?
Alternatively, you could try applying Normal style to the text paragraph and then reapply Heading 3.