Headliner not secure to car.
Headliner glue would fix that.
If the foam backing is still intact you can try squirting some 3M heavy duty contact adhesive to get it glued back up. If not it may just have to be re-done. I’ve done a couple and it’s not fun but not expensive. If you have someone do it my upholstery guy said he guarantees them to the first stop.
Sorry to hear that, you have my condolences.
The foam has failed. The entire headliner needs to be removed; the old foam removed; and new material applied with specialized adhesive. It is a pain and a mess. Have an upholstery shop do it.
Cheap “fix” is to use upholstery screws. These are short corkscrews attached to a plastic button. Available in sewing section of Walmart. Apply in a pattern that pleases you.
But it is only three years old. It takes a little longer for foam to deteriorate in the heat and sun. Pics would be nice but suspect the factory applied glue was a little spotty.
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I can’t tell if the headliner material is coming loose from the backing and sagging, or if the entire headliner with its backing is sagging from the original post. 3 years is a little early for either one. I just replaced the headliner in my 05 Lesabre. I took out the headliner and had someone else reupholster it, then I installed it. It was quite a pain in the butt, fwiw. But sagging headliners is kind of a pet peeve of mine.
I can believe heat, but sunlight? I doubt this car spends any time on its roof. 
I’ve never had any luck tryng to glue it back in place. The proper fix is to remove the headliner board completely from the vehicle (it is sort of a thick piece of cardboard-like material), put it on the workbench, scrape all the old glue and foam off, and glue new foam and material on, then reinstall the newly upholstered headliner board. On some hatchbacks the headliner board can be removed out the back, without major difficulty. But on most vehicles at the bare minimum some of the seats must be removed. In some cases the windshield has to be removed.
My solution to this problem on my Corolla sedan is to hold the sagging headliner up with numerous straight pins. Easy to do, does the job well, albeit with some appearance compromises.
Headliner problems are a pretty common problem reported here. Suggest to use the forum search feature to see what others have found. Click on the search icon. this page, upper right.