There's a thread on that same thing right here:
http://www.pets.ca/forum/showthread.php?threadid=4054
Some discharge is normal. If your dog is three, and assuming you've had him since puppyhood, I would be concerned if this is the first time you've noticed it. That might indicate it is a change for your dog. A departure from whatever is normal for your dog is something to investigate.
My boy has this regularly and it is a greenish/whitish/yellowish colour. But it doesn't smell.