Why aren't there any triangular molecules?
There exists triangular molecule, eg. cyclopropane, but they are highly reactive due to ring strain.


  Ring strain results from a combination of angle strain, conformational strain or Pitzer strain, and transannular strain or van der Waals strain.


C-C-C bond angle in rings should be approximately 109.5° degrees which in case of small rings like cyclopropane, bond angles are 60° whereas tetrahedral 109.5° bond angles are expected. The intense angle strain leads to nonlinear orbital overlap of its sp3 orbitals. Because of the bond's instability, cyclopropane is more reactive than other alkanes. Since any three points make a plane and cyclopropane has only three carbons, cyclopropane is planar.

Due to this strain they exists in higher energy state and therefore Because of their high strain, the heat of combustion for these small rings is elevated.

Kindly see Ring Strain, it will help you to understand about these kinds of molecules.