They are three separate meetings for three separate treaties, held in the same building at the same time. The COP serves the 1992 Convention, the CMP serves the Kyoto Protocol and the CMA serves the Paris Agreement. At Antalya they are COP 31, CMP 21 and CMA 8.
Each treaty has its own membership, so each needs its own decision-making body. A country that joined the Convention but not the Paris Agreement sits in the COP and not the CMA. Both the Kyoto Protocol and the Paris Agreement say in their own text that their meetings happen alongside the COP, which is why one conference carries three names.
In practice this matters when you read outcomes. The headline decisions on emissions targets, carbon markets and adaptation now come from the CMA, not the COP, because they sit under the Paris Agreement. Reporting that says "COP decided" is usually shorthand.
CMA 8 at Antalya carries a particular weight: it opens the second Global Stocktake.