PS3 is a considered a failure by some because it didn't make Sony any money but instead lost them tons. It had a turnaround in fortunes but overall its main success was in keeping the Playstation brand relevant and cutting the losses significantly which put Sony in a good position to launch the PS4. In that sense it can be considered a small, yet significant kind of success. My point basically is that success isn't necessarily a binary situation where "greater than" or "less than" sales are the sole determining factors. I'm not saying the 3DS is a success...yet, but being behind the DS doesn't automatically disqualify it.
Losing half of your marketshare is a failure too, beyond money or anything. Sony losed they privileged position in the market like Nintendo did, they didn't only fail in "making money" they failed also at that. The fact 3DS isn't capitalizing all that market created by DS is a failure by itself, despict the fact that 3DS is making money for Nintendo (way less money that DS, that said).
It also missing all Nintendo forecast until now. Something is not going well, clearly.