release_date.xxx filters all release dates which is not what you're going to want. I would add a with_release_type filter to force the query to only look up theatrical releases:

https://api.themoviedb.org/3/discover/movie?api_key=###&language=en&region=US&sort_by=popularity.desc&include_adult=false&include_video=false&page=1&release_date.gte=2017-04-17&with_release_type=3

as suggest, I add with_release_type=3(basially copy pastes your code) but, still I can see the movies that already released like sings, boss baby, the founder(2016-11-24) etcs.