I can't see the issue with just making this a 3-hour movie. I think you can easily adapt the novel into a 3-hour film. Actually, the 1990 miniseries was a little over 3 hours too, and I think it's an excellent adaptation.
Look, you can't adapt this material perfectly, and it's the same with The Stand. But what you can do is look at the material and break it down to its most basic components - the essential core of the story, and that is this: 
In the 1950s, a group of 7 friends calling themselves "The Losers' Club" have to defeat an evil that is terrorizing the children of a small Maine town. After thinking that they defeated "It", 30 years go by and it is now the 1980s. "It" has returned, and the children, now grown up, have to defeat him again. 
What's wrong with that? That's the whole story. That's a 3-hour movie.