Wii does not have WiFi N either. N mode must have some limited backwards compatibility with G that the DSi just doesn't work with very well because I know that WiFi N was going to be NEW with Wii U and not on the Wii.Someone listing features made a mistake somewhere then on the 3DS. I have my router switched to N only mode and my 3DS can still connect while my dsi can not. All my device (except the psp and wii, which I don't play online anyway) have wifi N.
EDIT : and a quick google search seems to confirm that both Wii and 3DS have b/g adapters, no n. (the N doesn't stand for Nintendo if that's what's confusing you j/k)
I never said the Wii had N. Your really dumb if you think anyone thinks the n stands for nintendo. BTW welcome to my ignorelist.