98-99, 99-00, 00-01. It's not too uncommon. 54-56 had a nice streak. 73-76
http://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_v5.php
I think a 3rd Nina year would be weird because of Global warming bias right now. 1 center El Nino event since 2010 in this exponentially warming climate?