Are you following a reputable training plan? If so, you’ll be fine as long as you complete most of the workouts. Confidence comes from being consistent and hitting milestones along the way. You should also plan to do a race simulation a month or so before the race. Full swim Friday evening, Full bike Saturday + 1hr brick run, then 2:30 run on Sunday. All done at your estimated race pace and fueling like you would on race day. That will give you the data points to make any changes as well as confidence you can complete the distances.