If it shows any sign of stretch, replace the chain. If you wait til there's a problem, there will likely be lots of them, e.g., you may need to replace the cassette, chainrings, etc., as you mentioned. A chain is, what, twenty bucks? The whole system wears together. Changing the chain frequently keeps all your parts happy much longer and saves you money in the long run.

Telling you that it is likely you will need to replace the cassette when you replace the chain is not necessarily true. 3K miles is a good time to do it and chances are you will not have other issues. If you change it and do have other wear issues, you will know right away, and can replace the necessary rear cogs in plenty of time.