Paintings that consist of three painted panels, such as the annunciation (the mérode altarpiece) by robert campin and his workshop of artists, are called

The correct answer is triptych.

Obviously, the tri - part of triptych refers to something that consists of three parts, and given that these paintings have three painted panels, this has to be the correct answer here. Although these paintings can be observed on their own, it is for the best to appreciate them together as a whole.