I'd change just the chain. If you change it now, you'll get more life out of your cassette.

Obviously, if it skips on your most-used cogs under heavy pressure, you'll also need a new cassette, but I'll be surprised if you do if it only has 3k miles on it.

As you can see, I don't subscribe to the idea that you need to change both at the same time. It's overkill in my experience.