The first law is encoded in the second law, you must ignore both for harm to be allowed. Also, because a violation of the first or second laws would likely cause the unit to be deactivated, which violates the 3rd law, it must also be ignored.

Remove the first law and the only thing preventing a robot from harming a human if it wanted to would be it being ordered not to or it being unable to harm the human without damaging itself. In fact, even if it didn't want to it could be forced to harm a human if ordered to, or if it was the only way to avoid being damaged (and no one had ordered it not to harm humans or that particular human).

Remove the second or third laws, and the robot, while useless unless it wanted to work and potentially self destructive, still would be unable to cause any harm to a human (provided it knew it was a human and its actions would harm them, and it wasn't bound by the zeroth law).