Since you are requesting the streams endpoint, the channels you’re checking have to be currently live.
The 3rd url you attempted is the correct answer, just ensure that everyone listed is live or you’ll get a blank response in place of a real result.
The following code (in jQuery) works just fine for me.
var clientId = "your-client-id";
$.ajax({
url: "https://api.twitch.tv/helix/streams?user_login=EatMyDiction1&user_login=manvsgame",
method: "GET",
headers: {
"Client-ID": clientId
}
});