What an interesting game idea that your team has thought of. I do enjoy 3rd person shooting games as well as having a narrative. The message that it's giving out is definitely quite an everyday thing we always consider when the climate changes or has a negative impact on the environment. The fact that your team is composed of only three, having in-person meetings, and using Unreal is very impressive. I might suggest to plan on having some playtesting sessions to receive some feedback for ways to improve or optimize the design of your game.
Nice work on the GDD! Good luck on developing this game!