To be a series on Goodreads, books need to have more than a common theme -- like characters in common, an overarching storyline, a shared universe, etc.
Aside from common themes, the stories are all set in the same world and connect together in book 3. Storylines from book 1 and 2 cross over to book 3 eventhough all books can be read as standalones.
Series title: Quick Bites: MM Short Stories
Book 1: https://www.goodreads.com/book/show/2...
Book 2:https://www.goodreads.com/book/show/2...
Book 3: https://www.goodreads.com/book/show/2...
Thank you!