A shill is someone who pretends not to be associated with a particular company and promotes that company while on that company's payroll. It's possible that this description fits the first two; especially the 2nd one. However, the third of that trio doesn't fit the definition of a shill. He claims he's from the company and seems to legitimately be answering a couple questions/points. I think it's beneficial for members here when company representatives come here & answer questions for posters, providing they're providing honest answers.
Hopefully one of the other moderators with more experience in that area can look into it further; but the first 2 of those posters haven't returned. I'm not sure if a lot of companies really understand one point though: if those first two were shills for the company (and even if they weren't), that company needs to realize that things like that make them look bad. There are a lot of websites that are attempting to do search engine optimizations by posting legitimate looking replies in threads here. When that happens, not only is the poster banned, but often, the website names end up on the censored list here. I had searched for one yesterday to verify that it was just spam; I searched by the username. That particular URL had been removed from many forums where it had been spammed - people are getting smarter about these things. I just hope google is able to adjust the system by which it ranks websites - I'm tired of the first 30 hits being crap.