<RichTextBox Name="richTB">
  <FlowDocument>
    <Paragraph>
      <Run>Paragraph 1</Run>
    </Paragraph>
    <Paragraph>
      <Run>Paragraph 2</Run>
    </Paragraph>
    <Paragraph>
      <Run>Paragraph 3</Run>
    </Paragraph>
  </FlowDocument>
</RichTextBox>
