|Eat Snickers and go FAST!|
Like most things in life, your core isn't an issue until it becomes one. Improper movement patterns and instability are major culprits in creating injuries and having a solid core can remedy that. Do you have lower back issues from being hunched over the bike or have knee problems from bad run form? Many athletes lose efficiency when they fatigue and can't figure out why they can't keep the pace... A solid core may not be the answer, but it could be! If you've showed up to a race with an injury, you know that all that training really didn't work when it counted!
Here are my recommendations: Get a bike fit and learn how to ride with good form and proper ergonomics. Thank you to Dave Luscan for helping me with this. Learn how to swim correctly and for god's sake keep your head down. Pay attention to how you are running and have someone take a look at your stride. A good book is 'The Running Revolution' for you to develop some idea of what you should be doing. If you are checking all of the technique boxes and using the correct muscles to move you through the various sports, then you can go ahead and spend some time really focusing on developing those beautiful abs.
In closing, major improvements and injury prevention can be accomplished simply by being aware of your body during the day... In reality, you can say you should be working on your core every waking hour of the day!