Hey there Exo, thanks for sharing :)
What stage in the writing process are you at with these? Are you still worldbuilding, character profiling, planning and outlining, or is this all done and you just need to start writing? Personally, when I had time to work on multiple projects I finished my planning for them all and then just started writing in short bursts for each, whenever inspiration struck for a scene. But you need to be way more organized with managing the projects in that way than I was at the time, so none of them saw the light of day in the end... But, if you can manage it, then you can work on all three and not have to choose! (Suuper-san is actually really good at this in my opinion -I recommend checking out his progress thread, wherever it is).
You haven't given us a lot to go on for understanding your plots here (although giving us the themes is something), and because your descriptions are so generalized it makes them seem a little generic story-wise and therefore hard to judge/get very interested in... That said, I found a kernel of something to be interested in worldbuilding-wise for each idea:
1) The standout thing for me in this one is human augmentations/cybernetics being taken from mechanical/metal aliens. It reminds me of X-COM a bit with how they reverse engineer alien tech to equip the soldiers with. The "what makes humans human" aspect is something that has been pretty well explored in Sci-Fi (Asimov's I, Robot being the prime example...), but I think there are still interesting options for you. For instance, you could throw in an element of humans using the tech but not really ever truly understanding it or even bastardizing it. Or look at it from the point of view from the aliens; maybe they don't like being hunted for parts, generating conflict? Perhaps you could go a slightly different angle, where the aliens are rather small metal creatures that enter into symbiotic relationships with the humans to confer the augments?
2) I have a soft spot for Arthurian aesthetics and legends. Is this idea based on a Pendragon campaign then? Also, I think this idea could stand out a bit if the world is already in a post-apocalyptic state when the story begins - this would set it apart from the myriad of other stories that are all based in standard European Medieval Kingdom settings with a "protecting the realm" plot.
3) Quite light on the details with this one. In all honesty this basically sounds like the plot of One Piece to a large extent... not necessarily a bad thing, but I think you'll need to lean on 1. a really vibrant setting, and 2. a really robust magic system that interacts with the setting, to set this idea apart. Not many stories focus on the crime boss though, no one of this particular disposition - the closest I've seen is probably Tsuna in Katekyo Hitman Reborn!, another favourite of mine. Once again, I think a really unique and integrated setting and magic system are required to make this one seem different enough.
So my advice is to research, conceptualize and plan for all of them, then get to writing bits and pieces and see what sticks. Good luck!