There are a lot of useful Sketch resources out there but I didn't find a good Default Bootstrap 3 Grid Template so I made one that really works for me.
It's just a simple sketch file containing 4 different artboards. No layers. Grids were made using the native layout grid (View > Show Layout) and some extra guides.
As I think a 12-column layout for Extra small devices is ridiculous, I made it with 4-column grid in mind. It's not 100% perfect...but it works :)