it showed The Rock in a dark office, and the girl who was undercover in Part 2 enters with a file and she says he might want to see this about a robbery in Berlin, and he asks if Dom (Vin Diesel) was involved and she says no, he replies he's not interested and she places the file open on the desk before him and says do you believe in ghosts, and there is a photo of Letty (Dom's g/f who died in the last one) in a car.
So it looks like there will be another one.