Have you done no training at all or just not much?
Have you any experience cycling, running, and most importantly swimming?

If you have no experience open water swimming, and this is an open water race, I would strongly advise you to reconsider for safety reasons. This is regardless of whether you are a strong swimmer, although I'd be even more concerned if you're not.

The answers to most of your questions are not simple, as different people seem to react very differently to nutrition solutions. The race is the wrong time to try using these products for the first time. I like to have plain water to drink. So I avoid energy drinks. Lots of people use lots of energy drinks. Different people find different concentrations palatable and digestible. I use gels only for mid distance and typically drink a little over a litre of water on the bike leg. You won't eat or drink during the swim and will typically get drinks on course during the run. You can bring your own gels for the run or grab whatever is provided on course. Your question about bringing 3L of water is concerning. You base this on a figure of 200ml/30mins, so you are suggesting you'd carry water to last 7.5hrs. How would you carry it, when would you drink it? This demonstrates that you really don't understand what you're doing yet. Sure, you have a week to figure out some of the theory, and make a plan, but you don't have time to practice any of it or figure out if it's realistic. Again my main concern is the swim. If the bike or run are much tougher than you expect, you go slower, you drop out, or perhaps injure yourself. But you can kill yourself getting into a mass swim totally unprepared. People can and do regularly panic in open water, even experienced swimmers. But it's much more common with less experience and and when it's crowded. Panic in the water is dangerous.

Thanks a lot for your answer!
It's a private competition, so there won't be any drinks on course. Because of this, I would store some drinks on course at the day before the triathlon and hope they are still there while the triathlon.
I will swim in a public swimming pool and I'm an experienced swimmer. I already tried the 1,9km and did it without a problem.
And I won't do the triathlon alone! I'll do it with a good friend of mine and of course we both take our mobiles with us in case of emergency. He also is an experienced swimmer (we both work as life guards).
We'll do the triathlon quite slow, the challenge is to do the distance, not to beat a time. So I think it is safe enough.
Of course anything can happen, but I think it's safe enough and if there is any problem, another friend can help us in less than 15min.

2017 I did a lot of running and some cycling but since 2018 it was less endurance training and more weight training

Ah okay. Well being an experienced swimmer, and most importantly doing the swim in a pool it's a considerably different situation and I wouldn't be so concerned. At my age (early 40s), I'd be highly likely to end up injured if I tried doing that with no training, but at 19 you may well get away with it.

haha I hope I'll get away with it ;)

but back to my questions from the beginning.. which nutrition do you use and what food do you eat before/while a triathlon? Do you have a link to a good nutrition/food plan?
Or can you give me any other advices for the triathlon?
I would be very happy about that!

It sounds like it is not an organized event and just a couple of friends swimming, biking, and running, correct?

If that's the case, are you're bike and run courses 1 big loop, multiple loops, point to point, or out and back? Are there places to stop along the route to refill water? Gas station, convenience store, etc?

For food, I'd just eat stuff that won't upset your stomach both before and during the tri.

Good luck!

---

Strava

Yeah, its not an organized event.
The bike course is 3 loops, the running course is point to point.
I won't have any issues while biking, because I can store 1,5l in my bike (which should be enough).
There won't be any possibilty buying water for the running course, because the stores are closed that day. I'll store something the day before or call a friend to bring us something with his car.

Thanks a lot for all your advices! Your help made this triathlon even possible for me.

I'm proud to say that I did the triathlon today! The time was 8:03.