That displays thousands, mills.

Yeah, when you put displays next to each other, they automatically sync up. Connect the wire to the right-most display.

Yes, I understand it. But if I have 1 piece of 3 displays together I can put 999 or 999.999 in one or two triple displays

I'm not entirely sure what you are asking. The game's circuit network does not allow for decimals, so 999.999 is just not a thing (Unless they changed that very recently).
If you put 3 of the big displays together, you can display 0 to 999. The small displays are two decimals each, so 3 of those together would be able to display 0 to 999999.

Thank you

He is talking about a visual appearence of a large number aka thousand separator.
if we want to use small nixie tube, we get numbers with a "hundreds separator", and not thousand.

for example, it take more time to read |4|02|18|00|, than |4|021|800|,
or how much is that: |50|00|00|00|00|? 500 mil? 50 mil?
I need to count zeros, that takes idk, 4 seconds, but the same number with the thousand separators |5|000|000|000|, it tooks milliseconds to read 5 billion.

therefore, small nixie tube (1 tile height) with 3 numbers in a row, maybe 2 tile wide would be helpful.

Yeah, I've actually wanted this as well for a while. It's in my todo queue to automatically merge them into a single larger nixie display, with an extra digit in between.

https://mods.factorio.com/mod/NixieTubesThreeDigits