Technically this isn't a three ring trick but a mixed prop trick. If you're looking for three ring tricks, though, you may find it useful.
Performing a Saturn is simply throwing a ball through a ring while juggling, making (for that short, but beautiful moment) the two look like a very tiny, poorly scaled model of mighty Saturn.
If you want to learn more about Saturn, visit the wikipedia page here.
Tips/Hints: This video shows the basic (see:much easier) version of this trick with the ring at 45 degree tilt. If you're especially ambitious you can try different angles and heights to change the effect.
Another alternative method is to use two balls and one ring instead of two rings and one ball. The odd one out will be the one that likely gets the most attention from the audience.