I don't know about actual sales numbers wise, but Reach made the most money in the first day. Source.
I believe halo 3 has outsold in be good amount though in the end.
Edit: According to Wikipedia, Reach sold 3.3 million and 3 sold 3.7 million in the first month. Reach didn't have the same legs apparently. I'd imagine that was thanks to Call of Duty becoming the beast it is, and following up the mediocre ODST. I'm sure 4 will be a beast.
Pretty much. Halo 3 didn't have to contend with a fully-established COD juggernaut, COD4 was flying under the radar at that point, and it wasn't until mid 2008 that it took over as THE shooter. By the time MW2 launched, it wasn't even close.
Combine that with a bit of franchise fatigue (people had been playing Halo in some capacity since 2001... Reach came out in 2010), and there was no way Reach was going to pull in the same numbers long-term as Halo 3. It doesn't have much to do with game quality, which I would say is about even (Reach has more content, better single-player/co-op content, and divisive multiplayer).
I wouldn't be shocked if Halo 4 didn't reach Halo 3 numbers either. It may just be that people moved on from Halo in the same way they moved on from Quake before that, and will move on from COD eventually as well.