Bike is THE most important as it's the one you will typically spend the most time on in most events. It's the one that should be focused on for improvement if you are not already an accomplished rider. It will also keep you fit for run and swim. Left weights especially chest, shoulder and triceps to help if you can't make the pool. Biking will help your runnning, BUT, you will still need to spend time road running. If all you'll do are sprints - try and get 12-15 miles a week in. You'll need to do more than that if you are considering olympic plus distances.