There are a lot of cosmic characters fans would like to see in Guardians of the Galaxy Vol. 3 with Nova somewhere right at the top of the list. James Gunn has previously hinted that he could make an appearance there but he confirmed during a HASCON panel this weekend that there's at least one hero from the comic books who won't be showing up despite his ties to the original Guardians of the Galaxy.
That is Vance Astro, the team's leader in the source material. He volunteered for a government sponsored trip into the cosmos and ultimately got himself some super powers and became the leader of a team featuring the likes of Martinex and Charlie-27 (both of whom appeared at the end of Guardians of the Galaxy Vol. 2). Gunn has said that team will have a key part to play in the Marvel Cinematic Universe moving forward but he clearly has no intentions of having Astro join the group.
Asked what the chances of Astro showing up are by a fan, the filmmaker replied: "Not great. I’m done with [the script’s] treatment." He'll obviously start penning the screenplay soon and we can discount one character from the threequel. On the plus side, Adam Warlock is definitely going to appear!