I recollect having discussed this last year with an adcom member from Kellogg. What she said was that with R3 applicants, they're essentially looking for exceptional applicants and/or those with unique profiles. At R3, they're essentially looking to garnish the class.
I'm not sure if they're biased against international applicants, but I'm guessing they'd be wary that there might not be enough time for international applicants to get all the documents (financial aid,visa, accommodation, etc.) ready in the little time that'd be left before the start of the program.
Net Impact's BUSINESS AS UNUSUAL Guide - a must-read!