Hi @Koepel,
Thanks for such a detailed help, 
Will answer inline with bold.
The sensors don't take a lot of current and still work with 3.3V. The Arduino does not have a problem with powering those sensors.
Can you measure the 5V pin ? Is it at least 4.5V ?
For the 5v pin i got:
5,04 V without resistances
1 resistor -> 5.04V
2 resistors - > 5.04V
3 resitors -> 4,89V
Where did you buy the DS18B20 ?
The real ones are okay, but the cheap ones are counterfeit and the cheap "waterproof" ones are only waterproof for a day, or a week.
I think i buyed the real ones: Waterproof DS18B20 Temperature Sensor - 10m | Sensores de Temperatura (ptrobotics.com)
The most important wire is the GND wire from the Arduino board to the breadboard. Do you have a single core wire that fits ? Or try two wires.
For the GND wires i tried too with two wires i got same behavior.
You could add one or more decoupling capacitors on the breadboard at GND and 5V.
I don't have this, will test when buy it.
How long are the wires ?
Of the sensores is 10M..
Is your final project one long stretch of wire, or a star-configuration around the Arduino board.
With a star-configuration, you can make more than one 1-Wire bus. Even a 1-Wire bus for each DS18B20 is possible. That causes no problem in the sketch.
Can you give me one example of an star vs one long stretch of wire? .I tried to be only one wire. I done some fast schematic, to exemplify:
Maybe the solution is Decreasing the pull-up resistor value on a single serial bus line when have more than 3 sensors.