Pikmin 3 has previously been billed as a 'launch window' title on Wii U in the States, meaning it will launch before March 31st. New information from Nintendo's recent 'Iwata Asks' panel narrows down the clues a little.
Nintendo Life reports that during the 'Iwata Asks' panel, Pikmin 3's release window was stated as 'Spring 2013'.
The site correctly points out that March 31st is eleven days into Spring, so this suggests the game will launch at the end of March. Take this as speculation for the time being, but it's nice to see Nintendo thinking about when it's going to launch the game.