The index card has three words scrawled on it. They are three high priorities. When the highest is not being worked on, then one of the others will take its place.
I, too, have long used a 3×5 card to drive my days in a similar way. Every day, I put my most important three tasks on one and then draw a line and use the portion beneath to capture incoming tasks and for scratch notes. I use one side of a card per day (so a single card can be used for two days). Been doing it for at least 10 years. I love them.