Simple enough:
<TextBlock Name="TextBlockWithHyperlink">
<Run Text="{Binding Path=UrlMessage2, Mode=OneWay}" />
<Hyperlink NavigateUri="..." RequestNavigate="...">
<Run Text="{Binding Path=UrlMessage, Mode=OneWay}" />
</Hyperlink>
<Run Text="{Binding Path=UrlMessage3, Mode=OneWay}" />
</TextBlock>