This is the idea that for difficult subjects, or for something you really want to remember, you aim to create three cards which will tackle the problem in three different ways with the same learning outcome.
You could create a Draw! card, a Simple card, and a Missing! card
You could create 3 cloze tags
You could compose a function with 3 different examples
You could create a simple, intermediate, and advanced problem
This is the idea that for difficult subjects, or for something you really want to remember, you aim to create three cards which will tackle the problem in three different ways with the same learning outcome.
And so on.