While the 3DS is of course not selling as crazy as the DS did... it's still doing very very well.
The fact of the matter is that 3D is and always will be a gimmick.
However, globally, the Nintendo DS is the number 1 selling platform period at over 150 Million at last count... The 3DS is the current top seller right now.
So I don't know why you think the DS is failing or that the 3DS is failing... They are making Nintendo a shit ton of money.
Week sales of Dec-10-2011 Worldwide
Platform Americas Europe AP/MEA Global
3DS 558,670 306,017 461,566 1,326,253
Xbox 631,870 192,453 27,060 851,383
Wii 453,288 262,882 98,047 814,217
PS3 287,263 291,658 121,494 700,415
DS 181,730 129,831 33,021 344,582
PSP 87,718 87,215 89,084 264,017