~서 , ~니 , ~니까 ????

[Hilal]

hey ! ive got a problem about ~니 , ~서 and ~니까 can someone explain these ?? whats the difference ?

피곤하니 쉬자.
피곤하니까 쉬자.
피곤해서 쉰다.

kinda confusing 도와주세요 !

and also 거든 is like them i think , i would like to know the difference