API specific post
If you use the API to collect / add messages, this will remain for you. A simple cycle / count should be sufficient to determine the third message.
Specific mail with topics
If you create a Tumblr theme, there is a theme operator / block to satisfy this:
{block:Post[1-15]}
{/block:Post[1-15]}
Displayed for a message with the specified offset. This allows you to insert an advertising or design element in the middle of your messages.
OP 3- , {block:Posts}{/block:Posts}.
{block:Posts}
{block:Post3}
{/block:Post3}
{/block:Posts}
-: http://www.tumblr.com/docs/en/custom_themes#posts