There exists triangular molecule, eg. [Cyclopropane][2]. But they are highly unstable due to [Ring Strain][1].


>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][1], it will help you to understand about these kinds of molecules.

[1]: http://en.wikipedia.org/wiki/Ring_strain
[2]: http://en.wikipedia.org/wiki/Cyclopropane