Mass Effect 3 gone from Origin's library

I doubt this will help, but... In Origin, upper left corner - Origin/Reload My Games. Maybe that option is there for a reason..

Already did that, the game still doesn't appears --'

Are you sure you logged in with the same credentials (email / password) you used when you first installed Origin and the ME3 Digital Deluxe Edition?

Other than that we cannot help you in account related issues, only official EA support can: https://help.ea.com/ You might have to switch your country to United States at the bottom of the page for all contact options to be available.

I'm sure I'm logged in with the same credencials that I used to buy the game, when I look at my order history, I can see the game, the Mass Effect 1 and Dragon Age: Origins that I bought together with it.

Even if you switch to "show all games" or "show hidden games" in Origin? That's the only remaining thing I can think of. But I'm afraid you'll have to speak to a game advisor as linked above.

Yeah, it still doesn't appears, but the main problem is that when I open it outside Origin, Origin opens and asks for the Serial and when I put it it says it's already been used 

Which solution did you get...i have the same problem!!