conniedodgen,
There is no set time to take sutures out. At three weeks, the wound should be well healed. Sometimes, when sutures are left in for an extended period of time, they can become buried. That just makes them more difficult to remove, but does not really affect the wound healing any.
However, if you are having cast problems, that need to be investigated. Anytime a patient complains of cast problems, if should be taken as serious enough to look at. A cast should always be comfortable. They are a pain to deal with, but they should not be painful. So, if you are having problems with your cast, contact your surgeon.
If you are still nonweight bearing, there are several devices that are available for you. A Knee-Walker/Knee-Scooter may help you get around more easily, if you have trouble with upper body strength. The freedom crutches come in a variety of styles. The basically place a device on the leg, at different angles (depending upon the variety), which offloads the foot, and places the weight bearing on the shin or knee. Some patients find them very easy to use, while others find them awkward and off balancing. All you can do is try them and see if they work for you.
Good luck.