Not at age 3, just free play.

For instance, about swimming classes for kids. I'd recommend just taking your kids to the pool, regularly, in the summer, and letting them play around.

With 1 kids I did swimming classes, and it was a cold, damp, miserable experience. Not to mention the commute there.

With another, we just went to the pool or lake regularly in the summer. We had fun in the sun, played around in the water with toys, and they definitely learn enough to be safe in the water fairly quickly. If you give one or two hints and nudges about swimming technique, they'll learn to swim.