Yep, you can have up to three Wonder Cards in your game. I'll use a personal example: My Pearl currently has Wonder Cards for Manaphy, Arceus and Shiny Pichu. I've picked up the Manaphy but have yet to pick up the other two. If I wanted to get Jirachi I'd have to delete the Manaphy Wonder Card to pick the Jirachi one up. If another event were to happen afterwards, I'd need to pick up Arceus so I could delete its Wonder Card to make room for the new one.Okay, another question. Is it possible to overlap mystery gifts? Like, if I decide to wait to pick up my Jirachi until I have a good movepool/nature decided for it, and I've gotten at least somewhat decent at competitive battling, but there's also another mystery gift available, can I activate the second one and still pick up my Jirachi afterward?
They're picked up in the order they were recieved.