In an interview with IndieWire, while discussing the upcoming Guardians of the Galaxy 2, writer/director James Gunn revealed that he is already thinking about Guardians of the Galaxy 3:
We open up on a Mission: Impossible scenario where all the characters die… No, to be honest, I had the basic story for number two while I was working on number one, and perhaps even beyond that. So it’s gonna answer a lot of questions that are proposed in the first one, and we’ll be able to get to know some of the characters we didn’t get to know in the first one a little bit more, and we might meet a few new characters too.