"Long term" trends were discussed recently here:  

https://www.climate.gov/news-features/blogs/how-pattern-trends-across-tropical-pacific-ocean-critical-understanding-future

The trend in the Nino3.4 region is close to zero since 1980.  If you want 30yr climatology/averages, then you can get those numbers from this plot (using ERSSTv5 data):

 https://origin.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ONI_change.shtml

Toward the bottom of this page is a link that gives you the Nino3.4 climatology numbers in the 4th column.  Hope this helps!