At the top of any Forum page is a link: are you getting paid to post.
When you click on that link, you will be taken to a page that explains what the requirements are for the $3.00 posting bonus. You've probably scanned through that, if not, read through it.
In the middle of that page is a green link that will take you to the page where you enter 3 ID's from the 30+ posts that you've made in December.
On the right side of that page is a reference to how many posts you made so far this month. No guarantee that all of them qualify. (Posts to the Contests and Promotions section, for example, do not qualify for the posting bonus.) If your number of posts are 30 or more, click on the link below the count to see the posts you've made.
In the header above each post is an ID number. Select 3 posts that you like (feel are your best or most responsive) and write the ID numbers down on a separate piece of paper. Then go back to the previous page (usually on another tab), and enter the ID numbers in the box and click submit.
Make sure you pick up December posts only. FC will check all of your posts to make sure you have 30 meaningful posts. As you go through them, if you decide there are some that aren't meaningful, you still have 2 days to post some more.
I usually have 40-50 posts before I submit my request.