When 3 green leds are in series, they all light up. When they are in parallel (after a 220 ohms resistor), they don't light up.
I don't understand why. Any idea?
I just asked ChatGPT and I understood:
- in series, each led gets the same current (23mA)
- in parallel, that current is divided between the leds, and that's not enough for the leds to light up
Solution: put that resistor on each parallel branch, so it limits the current only for that led, or use a smaller resistor.
They should light up.
If the 3 LEDs are from the same original batch, they should divide the current between them, roughly, but never quite exactly, equally. Each will only be 1/3rd as bright as they would be if only one was connected.
If the LEDs are not from the same batch, they may not be very evenly bright. One or two might not light at all.
So it's not clear why your parallel LEDs did not light.
Nonsense. Typical of Chat GPT.
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At least one should light up. Did none of them light up at all, or were they just dim?
I'm betting the poster has series and parallel confused, because if you just swap those two words in the OP, everything would make sense.
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Maybe not.
Green LEDs have a "forward voltage" of around 2.2~3.2V, depending on how they were manufactured. If you connect them in series, that's a total of 6.6~9.6V. If you supply only 5V from an Arduino, they should not light, in theory. But perhaps you were supplying 9V or something.
EDIT: I think @camsysca figured out the problem!
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Thank you all for answering. I tried again, and it looks like I got parallel and series confused, indeed 
Sorry. The leds in parallel light up, the leds in series don't.
In parallel, as Paul said, the current is divided and each led lights up about 1/3 as bright.
But why don't they light up in series then? The current is not divided. But the tension is, that's why?
I attached the two circuits for reference.
As @PaulRB intimated, to have current flow you must achieve each of those voltages "in series". IF you start with 5V, you haven't enough voltage to do so; if any one of the LEDs does not conduct due to that, then you have no current flow through anything in the series circuit.
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I see, thank you very much! 
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