If there's 3 ads, it will rotate through the 3. If you want different ads for the top of the page and the bottom - no problem. Just run 2 copies of the ad and different directories. Of course you can go hog wild and buy yourself a sophisticated ad script but to start out any good simple random ad script will work just fine. Monitor the performance Check often and make changes if needed.
Ads grow stale.
Do you visitors respond to text links or banner ads. What's new that you could be adding. Add content and tools can really help attract traffic and hold an audience. I can't emphasize this one enough. Take a unique approach on your site and then add good strong related free content and services.