The one in GTA3 isn't really much of a "hidden message" as that's just the name of the Liberty City baseball team in that game (it's mentioned on the radio multiple times, after all).
These were hilarious, especially the Sonic and Mario ones lol. Also if your looking for a game FULL of easter eggs try MGS4, like when your fighting the Beauties/Beasts in there bare female form and you look at them through the camera they pose like models.
here's a link to show any that care:
http://www.youtube.com/watch?v=BCvnoggWQvo