That would be the "Charting Outcomes in the Match" available here:
http://www.nrmp.org/data/chartingoutcomes2007.pdf
What the data showed was that the #1 key to determining if you got into your desired specialty (they defined desired specialty as the field you had in your #1 spot on the rank list) was how many programs in that specialty you ranked contiguously (ie 1-n until reaching a different specialty or the end of the list).
The median number of programs ranked for successful U.S. Seniors in peds was 8. But if you go look at the tables, there were 1691 US Seniors who matched in Peds and only 46 that didn't...which is only 2.6%.
If you look at the breakdown of how many programs were ranked contiguously, and whether people matched or not, it's really, clear that for US seniors, it's pretty rare to not match. No matter how many programs were ranked, the match rate was always >85%
This data is much less true for independent applicants, but the data is skewed because of the large number of suicide match applicants (those who only ranked one program in peds).
So basically, if you are a US Senior, there's little to worry, about.
For comparison's sake, if you look at something like Derm, there's really a significant difference in outcomes based on how many programs the person is able to rank. Once above 7 ranked programs, the liklihood of matching in this very competitive field greatly increases - which intuitively makes sense; if you can manage to earn 7 or more interviews in the first place, you're probably a very strong candidate to begin with.
As far as getting top three choices, not sure if I've seen anything peds specific, but for the match in general, it usually runs around 85%...84.6% in 2008 (of the people who matched which is usually 93-4% or so for US Seniors...94.2% in 2008). If you include 4th choice, it's 89.6%. But again, nothing specific for peds.