The first one implies that people either think that their employer should cover part of the health insurance costs (as said in the original sentence), or they think that the employer should pay 100%. This is not correct because there are other possible opinions people can have, like thinking that the employer shouldn't pay for anything, for example.
The second conclusion is invalid for the same reason: it implies that people can only either think that the employer should pay a large part, or that the employer shouldn't pay anything. It is not considering other options.
The third conclusion does not work either because it is referring to what people think about <em>the amount </em>of the costs themselves, whereas the original topic was <em>how</em> they are paid for.
