It is hard to say because we have not always had operational monthly predictions of ENSO.  Here is an archive of our forecasts going back to 2001:

http://www.cpc.ncep.noaa.gov/products/expert_assessment/ENSO_DD_archive…

And here is the ONI archive of ENSO episodes:

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ens…

In that very short time span (relative to the history of ENSO, which has lasted for millions of years), CPC has never declared onset during March.  But this is relatively meaningless because the ENSO cycle has varied over time.  Even our observational record of Nino-3.4 index values going back to ~1950 is too short to really know for sure whether a March onset would be unprecedented.   It is also not yet clear -- based on soley the ONI -- whether it would be sufficiently distinct from the warming that began in late 2014.  

Below are our current probabilities for La Nina during 2015.  While they are not zero, they are smaller than the chances of Neutral or El Nino:

http://iri.columbia.edu/our-expertise/climate/forecasts/enso/current/?e…