Pandacoin #1 was just another altcoin.

Pandacoin #2 was the brainchild of wolong - the asian market manipulator who is credited with manipulating dogecoin, uno and others. This coin is modified vertcoin so it needs a custom miner to run. It has widespread developer support and help from the unobtanium coin devs. It is garnering lots of attention due to the attachment to wolong who stands to become a multimillionaire if the coin is successful. It was timed to launch the day dogecoin payouts halve giving miners something new to pile into.

Pandacoin #3 is an attack by 4chan users to get back at wolong for his market manipultion techniques and perhaps taking more credit for dogecoin than he should. The plan is to confuse users over what is the real pandacoin.

http://www.cryptocoinsnews.com/2014/02/14/notorious-market-player-wolong-releasing-pandacoin-today-4chan-declares-war/

This should provide some interesting drama! Personally I am mining coin #2 as wolong can generate attention and stands to make a lot if it is successful.
#1 is generic and #3 is run by 4chan who don't usually have the best of intentions.