Buddha trinity is the set of three Buddhas, Buddha Shakyamuni, Buddha Amitabha, and Medicine buddha. even with a deep study regarding why these three buddhas are manufactured as a set. we have not found any religious backing story for these three buddha to be in the set.
Hence we are assuming this has more to do with the popularity of these three buddha individually, hence the manufacturer make these three more frequently. which now has transitioned into becoming a set.