$\mbox{SU}(3)_L \otimes \mbox{U}(1)_N$ and $\mbox{SU}(4)_L \otimes   \mbox{U}(1)_N$ gauge models with right-handed neutrinos
  Pisano and Pleitez have introduced an interesting $\mbox{SU}(3)_C \otimes \mbox{SU}(3)_L \otimes \mbox{U}(1)_N$ gauge model which has the property that gauge anomaly cancellation requires the number of generations to be a multiple of 3. We consider generalizing that model to incorporate right-handed neutrinos. We find that there exists a non-trivial generalization of the Pisano-Pleitez model with right-handed neutrinos which is actually simpler than the original model in that symmetry breaking can be achieved with just three $\mbox{SU}(3)_L$ triplets (rather than 3 $\mbox{SU}(3)_L$ triplets and a sextet). We also consider a gauge model based on $\mbox{SU}(3)_C\otimes \mbox{SU}(4)_L \otimes \mbox{U}(1)_N$ symmetry. Both of these new models also have the feature that the anomalies cancel only when the number of generations is divisible by 3. 
Robert Foot, Hoang Ngoc Long, and Tuan A.Tran