Variable modified by different threads.

[Ritina via Digitalmars-d-learn]

How can I implement a program where I have a global integer 
variable g, and three threads: the first thread increments g by 
1, the second thread increments g by 2, and the third thread 
increments g by 3? Additionally, while these threads are running, 
I should be able to access the value of g both inside and outside 
the threads.