€3 will just get you a seat. Not recommended for a good nights sleep. The couchettes and sleepers will cost more.

 There’sa good guide to reservation fees here

https://www.seat61.com/interrail-and-eurail-reservations.htm

so basiclly with my global pass i need to pay extra for sleepers (100 euro) and for a seat (3euro), right?

i think i get it, when i have global pass and want to buy on oebb.com i only choose a option with reservation because my pass is de facto my ticket to this ticket, if i wanna sleeper i need to pay extra to my global pass!

Yes, you’ll always need to pay extra for sleeping accommodation. 
 

If you just want a seat, it depends heavily on the train. Sometimes €3, sometimes it doesn’t require reservation- just get on the train. Sometimes it’s more expensive. The Seat61 guide tells you, country by country.