Except that Cade had a meeting with his great granddad's Force Ghost, so that right there implies Luke got killed.
Ya edited my post, though it doesn't say how he died though. Luke could have just passed away peacefully or some such nonsense by something. Vague deaths for all 3 characters, would be the best bet to go about them. Because no one is going to like the deaths, if they are described in detail. I mean look at Chewie, took a moon to kill him yet people don't like that he was killed.