Defining "harm" and in particular, "allowing harm via inaction" in any meaningful way would be difficult. For example, should robots spend all their time flying around attempting to prevent humans from inhaling passive smoke or petrol fumes?
In addition, the interpretation of 'conflict' (in either rule 2 or 3) is completely open-ended. Resolving such conflicts seems to me to be "AI complete" in general.
Humans have quite good mechanisms (both behavioral and social) for interacting in a complex world (mostly) without harming one another, but these are perhaps not so easily codified. The complex set of legal rules that sit on top of this (polution regulations etc) are the ones that we could most easily program, but they are really quite specialised relative to the underlying physiological and social 'rules'.
EDIT: From other comments, it seems worth distinguishing between 'all possible harm' and 'all the kinds of harm that humans routinely anticipate'. There seems to be consensus that 'all possible harm' is a non-starter, which still leaves the hard (IMO, AI-complete) task of equaling human ability to predict harm.
Even if we can do that, if we are to treat as actual laws, then we would still need a formal mechanism for conflict resolution (e.g. "Robot, I will commit suicide unless you punch that man").