Even before the release of Guardians of the Galaxy Vol. 2, James Gunn has confirmed that there will for sure be a Guardians of the Galaxy 3. He tells Complex:
There will be a Guardians 3, that's for sure. We're trying to figure it out. I'm trying to figure out what I want to do really, that's all it is. I got to figure out where I want to be, what I want to spend the next three years of my life doing. You know, I'm going to make another big movie; is it the Guardians or something else? I'm just going to figure it out over the next couple of weeks.
This should come as a surprise to absolutely nobody, since it's long been understood that Disney loves money and hates the not-having of money.