What is a good time for a beginner runner to run 3k? Watch
Keep at it!
Rather than stress about the time taken to run 3k look to just gradually increase the time you run for: 25 mins, 30 mins, 40 mins etc. If you have only been running a few weeks then you will definitely be able to reduce your time for 3k after a while but you will probably be more interested in your times for 5k and 10k by then as well.
Also build up gradually so as not to end up damaging your unconditioned knees.