Guys i finished shadow of chernobyl. However do i need to play clear sky ?
Cann i play with call of chernobyl every story mode of all three games ??.
Or how does it work ?

Hello :-)
Call of Chernobyl is a freeplay mod, it connects all the maps of the three games together into one giant sandbox. It does not however feature a story, well there is a "story mode" but it's very limited. If you wanna experience the story of Call of Pripyat, play that game instead (possibly with the ABR mod to easily improve visuals).

Call of Pripyat's story continues after Shadow of Chernobyl. Clear Sky is a prequel to SOC. If you wanna play Clear Sky, install the Sky Reclamation Project first. It's a community patch that fixes many problems with the game.

In case you'd like to see what Call of Chernobyl looks like, I'll shamelessly promote the let's play series I made a while ago (I still wanna do part 5 when I find the time): btw. you should get version 1.4.22. of CoC 1.5 was never finished and does not work properly.
https://www.youtube.com/watch?v=dNLHmU-nnzU

what about the complete mod ?

Short answer: NOOOOOOOOOOOOOoooooooo

Long answer: The Complete Mod uses really old versions of the fixes and improvements, and introduces some gameplay changes that make the game too easy/unrealistic. Such as reducing enemy range of vision to like 100 meters, even though you can hit them from way further away. Basically there are now way better alternatives out there to the complete mod that are just as easy to install.