There are a couple of them, though it depends on what you consider "serious" or not. I think Godot as it is now is reasonably capable for 3D (adequate for what

There are a couple of them, though it depends on what you consider "serious" or not. I think Godot as it is now is reasonably capable for 3D (adequate for what most small indie devs would want to do at least) but it is definitely behind Unity/Unreal in featureset, performance, and availability of 3rd party modules. And of course everybody has already spent years learning the other engines. 4.0 should be bringing a lot of major features and big performance improvements but that's a fair bit away. i.e. https://store.steampowered.com/app/992860/Intrepid/ https://store.steampowered.com/app/992860/Intrepid/ https://store.steampowered.com/app/467090/A_Game_of_Changes/ https://store.steampowered.com/app/467090/A_Game_of_Changes/ https://store.steampowered.com/app/818620/Music_Boy_3D/ https://store.steampowered.com/app/818620/Music_Boy_3D/ https://victordelima.itch.io/way-out https://victordelima.itch.io/way-out still in development: https://store.steampowered.com/app/824090/TailQuest_Defense/ https://store.steampowered.com/app/824090/TailQuest_Defense/