Regarding Loki, while it seems the inclusion of the Asgardian Trickster is a no-brainer, Tom Hiddleston recently questioned if he would be returning as he stated he hasn't heard anything from Marvel in years; hasn't heard about Thor 3, and also wondered if the fans wanted him back.
Now it appears we can all breathe a sigh of relief as Mark Ruffalo fills in USA Today that he has spoken with Thor 3 director Taika Waititi, and while Ruffalo doesn't know story specifics, he does state Hulk, Thor and Loki will be in the movie.
I don't know what the story is yet, but it'll be (Loki) and Thor, and then I'll be in there here and there. But I don't know any of the particulars quite yet. I've talked to Taika a little bit and I like where it's headed. ... Chris (Hemsworth) and I have a really good time together, we goof off and play around when Tom Hiddleston is around. So it'll be a lot of funny back-and-forth, especially with Taika, he does that really well. It'll have that kind of antagonistic Odd Couple-thing going on.
As Hiddleston previously stated, it's possible Marvel simply hasn't informed him yet of the Thor 3 plans. Ray Stevenson and Zachary Levi, two members of the Warriors Three, haven't heard from Marvel as well, but are expecting the call: "I’ve heard that it’s on the cards,” Stevenson told IGN. “As far as Volstagg is concerned, should he be called with his other Warriors Three to protect young Thor again, within a heart’s beat he will be there. So, I am waiting to hear. I’m very excited about the whole thing actually.” Likewise, Levi told CBR: “No, not at all. As far as I know, they are still writing the script. Once that’s all buttoned down, I’m sure they will be making the phone call to whatever actors are involved."
The USA Today article also makes mention of the Hulk appearing in Avengers: Infinity War:
...he will resurface in the two-part Avengers: Infinity War sequels...
And Ruffalo comments on the possibility of a standalone Hulk movie:
"Actually, it feels even further away. It's not Marvel's property, it's Universal's property. I don't know. It seems really problematic."
"Thor: Ragnarok" has a November 3, 2017 release.