How long does it take to ride a bike 3 km?

There's no set answer to that question, as it would depend on how fast you're going. And that would depend on quite a lot of things, how fit you are, how hard you're trying, what kind of bike you're riding, uphill/downhill, headwind/tailwind, if you're carrying any gear, if it's a sprint or a part of a longer ride, road/offroad....

Under most circumstances it'd probably take around 8 minutes, although a real pro under ideal conditions could pull it off in around 3 minutes on level ground. I could probably get it done in around 5 minutes.