Final answer:
The Summer Triangle's three bright stars are part of the constellations Lyra, Cygnus, and Aquila.
Step-by-step explanation:
The three bright stars that mark the points of the famous Summer Triangle are located in the constellations of Lyra, Cygnus, and Aquila.
Therefore, the correct answer to which constellations the three bright stars of the Summer Triangle belong is a. Lyra, Cygnus, and Aquila.