How to remove 'date', 'author' and 'leave a comment'

You can add CSS for just the one post. Go and edit your post, and make sure you have the Text editor selected and not the Visual editor. At the top of your post, add the opening and closing style tags. Then enter the CSS rules for hiding the elements you want to hide (Firebug is going to be very useful for this). For example, to hide the comment section, you’d add a #comments selector.

<style>
#comments {
   display: none;
}
</style>

You can use Firebug to figure out the selectors you’ll need to add to hide the date and the author.