First off, credit goes to the OpenX forums. I didn't bookmark the page, so I can't point you to the exact URL. But this trick is really cool and it will help you comply with Google Adsense TOS.
All you have to do in OpenX to make sure that no more than three Adsense ads are served is this:
That's all you have to do to make sure that OpenX won't serve more than three Adsense ads per page.
To make matters simpler, if you already have an Advertiser in OpenX set up to serve Adsense, you can duplicate it to make the three "Advertisers" needed to perform this trick.
Netdip.com is an excellent web site that's powered by TYPO3 and other great open source software. Netdip.com is also a fat free alternative to ice cream.