and in that package for example it clearly states that the company provides 401k plan with 3% match including Immediate eligibility and immediate vesting.

Then, once hired, either they tell you that you're not eligible for a year or that there is no plan for you at all? I can't imagine benefits are a legally binding document, and even offer letters can be later adjusted by raises or demotions. But if you send the benefits doc along with the offer letter, isn't that a commitment of some kind?