i work in motor insurance claims. its in our policies that we can take up to one month before settling theft claims. This is in to give some time in case the car is recovered and also to give time to carry out any necessary investigations. So three weeks dosent sound that long and 10 working days is not the norm for any insurance company for a theft claim. As Ravima has suggested just ask them why it has taken this long. It could simply be a case that they are busy and nobody has actually processed your claim yet
The company that I work for have a 21 day waiting period to allow for recovery. Typically the majority (if not all) of the paperwork can be resolved during this waiting period so that once this time is up, the insured can be settled straight away - chq should be with you within a week.