I had corrugator muscle removed along with upper and lower lids surgery. Why do I have 2 hard knots on my forehead at brow?

You had a pretty traumatizing surgery.  The body responds to this type of tissue damage with a robust healing response that ultimately makes enough scar tissue to heal the area.  An inflammatory nodule or knot would be pretty normal.  It would be appropriate to bring this to your surgeon's attention to be reassured.  It may take several months for this process to resolve but one would expect the knots to get better with time.

The hard knots you are feeling at three weeks after surgery is most likely  firmness from swelling from the surgery.  If you are concerned then it is best to see your surgeon.