Well, the lore of 3 would allow him to place an aspect the GM deems as normal difficulty. The limitations are whatever you wish. A fellow player in one of the PbP games here has an item that grants him the "Nimble as a Mountain Goat" aspect.
Keep in mind three things. First, 3 shifts is enough to declare an aspect, but not make it sticky. This means that he gets the aspect for a short while, but as soon as he tags it, it disappears. Second, the only real benefits of declaring an aspect are getting a tag (a single +2 or reroll) or invoking it with a fate point (and invoking won't really be available to him since it's not sticky). So while he could declare "Eyes Like a Cat" to get a +2 to a perception check in the darkness, it would only give him a single free +2. Third, aspects aren't just usable by you. If enemies can figure them out, they can use them too. If he makes a "Light as a Feather" aspect and jumps off a building, then maybe the bad guy who is pursuing him grabs him out of the air and pulls him back using that same aspect. And if he made an assessment to determine the aspect, he might even do it for free.