I actually play all three of those. I was into Pokemon as a kid, then YuGiOh, then Magic. Then anime club people got me back into YGO, and then a few of my close friends randomly decided to pool $50 to buy several boxes full of Pokemon cards from this one guy we knew. For Pokemon, we have our card pool that we just take whatever from when we want to do a tournament. For YGO, I have a deck I picked up for $15 that I've modified with cards donated to me by other friends and a deck a friend just gave me when he graduated. Magic, I've had the same deck for six or seven years now and it's been getting slight modifications over time. At this point there are eight cards left that I need - for now I proxy them - to make it complete. I mean, if I felt like dropping a grand on lands I could add in Bayou and shit but fuck that.
I also have a Cardfight Vanguard deck (somewhere) that I've never learned to play, and a deck of cards for Weiss Schwartz... which are all in Japanese so I don't even know what to do with them yet. :v