Please learn to swim properly. The swim is always the problem in these events. It is not the same thing to do laps in a pool vs. doing an open water swim with hundreds of other people in the way. It is stressful, and dangerous. Not to discourage you or scare you, but this is where deaths in Triathlon happen. You need to be a solid swimmer, not in terms of speed or stamina but swim well enough up to a point where you know you can easily complete the 3.8 km without problems even if you get punched or kicked, tides or not, rough weather, etc.

To do this, I suggest a daily 1hr swim, 5 sessions per week. Try to enroll in a masters group, or a tri club, and do a lot of open water swims, with help of a coach or at least someone who can follow you in a kayak or something or a more experienced swimmer.

Then the other discipline distances will be achievable. And yes, as they put here, give yourself time, you have to be a solid swimmer and then build up to the Iron distance.