Land: I am under the assumption that this would also be my primary battle morph. I'm gonna have to go with mountain lion. Stealth, brawn, speed, incredible senses, extremely deadly, and at least in my area of the States, something that wouldn't be too out of place.
Sea: Help me out here. I just don't know. My impulses say orca for some reason. I'm seeing a lot of prehistoric creatures on this list, but with only three morphs, stealth should be a possibility for all three. And being a megalodon or a badass ancient whale or sea-dwelling ancient reptile might win battles, but it would get noticed FAST. At the same time, I don't feel like orcas are fast enough, and they're not maneuverable in reef or complex environments the way a shark or dolphin would be. No idea. I don't look into sea life a lot.
Air: I wanted to say kestrel, but birds of prey are way too observable, and tire quickly if the air isn't working with them. It may be lame-sounding, but canada goose. They fit in just about everywhere, they fly fast, they have amazing endurance, and although they might not be able to inflict serious damage, they can still cause a lot of pain in a pinch. Alternatively, maybe some sort of swan; I KNOW that a large swan can kill a human.