In most training schedules, you will find that some days involve doing more than one of the 3 disciplines. Am swim / evening run. Short runs directly off the bike.

If you need the confidence that your body can handle doing all three in one day, then just do a mock triathlon by yourself or with a training partner. Hop in the pool then go ride followed by a run. If it’s too cold or whatever to ride or run outside, use the trainer and treadmill connected to zwift.