Without bothering to dig out my codebook I'd say no, that the NEC doesn't mandate 3-way switches in such a situation. In fact, you don't have to install wall switches at all if the lighting circuit breaker is SWD rated. Now there might be some other building codes besides NEC which address this but I personally am not aware of them. Were I faced with this I would either put 3 or 4-ways at each entrance or not use wall switches at all. I'm sure I'll be corrected if I'm wrong!