In the late 80s I read a fantasy trilogy base around Norse mythology. Its main character was the magician Surt and book 3 ended with Heimdall blowing the horn to signal Ragnarok and the start of a new world. I think it would have been written in the 80s, maybe the 70s.
This is a very long shot, but I wonder if this could be the Twilight of the Gods series by Dennis Schmidt. I only have the first book, and I cannot find much information about the other two books, so this is at best a tenuous identification but it does seem to match your description in many ways.
The series spanned the years 1985 to 1988 so the dates fit, and it is heavily Norse themed. Surt and his sidekick Jormungand feature heavily in the first book, The First Name, though Surt is a warrior rather than a wizard. What little information I can find about the third book, Three Trumps Sounding, suggests that it does indeed end with Ragnarok (or Schmidt's thinly disguised equivalent). I cannot find a copy, and the copy on Google books does not have the ability to search it so I can't check whether it ends with Heimdall blowing a horn.
I'm sorry this is so vague, but your mention of Surt immediately reminded me of the first book, which I do have, and given the similarities it's just possible this is the trilogy you remember.