Three weeks out is not very long, although it may seem like an age at the time.
Depending very much on your surgeon's rehab protocol/regime, will may still have considerable post-operative swelling in and around the joint. This can prevent full extension. Work on gaining this passively by one of two ways, but run both past your handling physiotherapist(s) first.
The first involves you lying on your stomach with your leg hanging off the bed/sofa from the shin downwards. The weight of the over-hanging foot and lower leg is all that is needed to help straighten the leg.
The second is even more passive use of gravity! Simply raise your operated leg and support it under the heel and ankle, leaving the knee completely unsupported. If you combine this with having your leg raised, you can not only passively work on your extension, you can also do ankle pumps to help reduce the swelling.
The swelling is your body's natural reaction to the trauma of the operation. Unfortunately the leg has no mechanism for pumping the fluids produced away from the trauma site, other than walking (the squeezing actions of the contracting and expanding muscles actually squeezes the lymphatic fluids back up the leg into the lymph nodes in the groin where they are re-absorbed by the body. By raising your leg, you are immediately helping; by adding ankle pumps to the equation, you are simulating the natural manner in which they would be taken care of. Think about what happens to your hands if you leave them hanging, without any movement for a prolonged period and how you then counter the ensuing swelling.
Beware of the comfortable cushion/pillow under the knee - it is a false friend
It feels great, but it actually works against the healing process. Believe me I wish I had known that when I had my first ACL reconstruction. I ended up having to have an MUA to break up the scar tissue that had "solidified" in there! so much for comfort
Be as mobile and pro-active as you can while resting is my best advice: heel slides using a plastic bag are also a neat idea for working on mobility. Too long in one position and the knee will stiffen up even more than usual.
There is an excellent article in the Information Hub which shows you different videos of each of the different exercises at various stages in a typical ACL rehab protocol. HOWEVER, do not try anything until you have OK'ed it with your PT &/or your surgeon
. Each and every operation is unique and has it's own peculiarities. Only your handling doctors and therapists know the whole picture for YOU.