Social media didn't exist then, hell the internet didn't exist then. It is hard it imagine how little information there was about this sort of thing.
When Halloween III came out all we had to go on was the poster and it was seen by basically no one in its cinema run.. It was mainly shown in drive-ins as most cinemas didn't bother screening 18+ rated movies.
So, no, no one really knew or cared who Myers was or that he was not in it.
The Halloween series and horror in general did not take off until home video did and that was still in its infancy.
It was probably at least 5 years after its release that I watched it.