http://www.vgchartz.com/article/87980/3ds-tops-5m-slower-than-wii-ds-ps2-but-faster-than-x360-ps3/Quote
Compared to other recent systems in an aligned region by region launch, the 3DS has reached 5m units slower than Wii, PSP, PS2 and DS but faster than X360 and PS3. If 3DS were to continue to beat X360 / PS3 long term and remain behind DS, Wii, and PS2 (but not PSP) that would put the device on a pace for 80m - 160m lifetime, although its way way too early to project that far out.
Figures to date for 3DS are actually most similar to DS so far, which launched weakly after holiday 2004 but ramped up substantially from 2006 onwards.
Sales Globally Weeks in Americas Weeks in EMEAA Weeks in Japan
Wii 7.76m 25 (2.95m) 26 (2.08m) 29 (2.73m)
PS2 7.23m 25 (2.28m) 26 (1.85m) 29 (3.10m)
PSP 6.59m 25 (1.75m) 26 (3.34m) 29 (1.50m)
DS 5.47m 25 (2.04m) 26 (1.01m) 29 (2.42m)
3DS 5.04m 25 (1.74m) 26 (1.45m) 29 (1.85m)
PS3 3.40m 25 (1.44m) 26 (1.02m) 29 (0.94m)
X360 3.17m 25 (1.76m) 26 (1.27m) 29 (0.14m)
Had 3DS launched in even one region over November / December (as each other system above did in at least one region), it likely would have reached 5m units in a faster manner - and probably ended up at 5m between the pace of PSP and DS.
Given 3DS software development costs between Wii and DS, and a hardware launch comparable to the original DS, the 3DS now looks like it is on track to be a worthy successor to the DS line, even as it continues to trend slightly behind it. Japanese figures in particular should be explosive for 3DS over December 2011 - January 2012, and encourage the migration of further major projects to the system in coming months and years. As those projects make their way westward, 3DS should be able to fully recover from its rough start just as the original DS recovered from abysmal Western numbers in early and mid 2005.