Hi. I have installed The sims 3 base, Sims 3 Late Night, and recently The sims 3 World Adventures, but when I load World Adventures (with disc in) a message appears saying "No game disc found! Please insert The Sims 3 ->LATE NIGHT<- game disc before proceeding." ... I dont know why it just ask for late night even though I have instaled WA too. Can someone tell me how to load game please?? thank you.

Game always load latest Expansion/Stuff Pack, no matter which order you installed it in, so since Late Night was released after World Adventure you need to have that disk in reader before starting game.

That may be because late night is the most recent expansion to come out, that you have installed, I think that's why it is asking for the late night disc even though you installed the world adventures one after, I could be wrong though

so if I have both and load Late Night Disc then both EP are already in game???

Yes....