Fix missing due date in exercise 3

docs/2021/exercise3.md

@@ -161,6 +161,10 @@ the system (e.g. invoking service methods).
 At the end of the exercise, do not forget to schedule an one-on-one meeting
 with a teaching assistant to demonstrate your work.
 
+The due date of this exercise is: **1 December 2021, 21:00 UTC+7**. Please
+ensure any updates to the fork repository related to this exercise were made
+and pushed before the due date.
+
 ### Checklist
 
 1. [ ] Choose one **complex function** from your group project.