Interesting factoid, according to wikipedia the PSP 2000/3000 is 6.7 inches wide by 2.8 inches tall by 0.75 inches thick. The WiiU controller is 10.5 inches wide, 7 inches tall and 1.8 inches thick.
That means that if we consider them as a bunch of rectangular solids, you could fit six PSPs inside a WiiU controller, in two layers of three each.
Unfortunately wikipedia does not have the dimensions for the 3DS easily on hand, but it does claim to have the Playstation Vita stats, 7.2 inches by 3.289 inches by 0.73 inches. Looks like you couldn't quite pull off the same trick with the Vita. The third sideways Vita would stick up past the top of the WiiU controller by 0.2 inches.