why are triple and double bonds less table than single?

There is a lot of of common misunderstanding here.

Between carbon atoms, a double bond has higher bond energy than a single bond, but less than twice as much. Likewise, a triple bond has a higher bond energy than a double bond, but the difference is less than the energy of a single bond.

With nitrogen, it is actually the other way round, which is why N2, with a triple bond, is so very stable and unreactive.

Textbooks often give "explanations" of this, by comparing the sideways overlap involved in a pi bond with the head on overlap in a sigma bond. I'm not sure I believe this, because of the different behaviours of carbon and nitrogen.

Triple and double bound are composed of a pair of sigma electrons and two or one pair of pi electrons . The pi electrons are located on pi orbitals which are not located between the atoms but above or under the plane of the molecules. This makes the pi orbitals easier to disrupt than sigma orbitals.the pi electrons react more easily with different functions, and are very reactive. The single bond are only formed of sigma electrons in sigma orbitals and are very stable