From the US Department of State website: ENTRY/EXIT REQUIREMENTS: PLEASE NOTE: We recommend that the passports of all travelers to South Africa contain at least two completely blank (unstamped) visa pages each time entry is sought.http://travel.state.gov/travel/cis_p.../cis_1008.html
I have never heard of the 3 pages rule and last time I was there I didn't have 3 pages blanks and had no problem.