If double and triple bonds are stronger than single bonds then why are unsaturated hydrocarbons less stable than saturated hydrocarbons?
When I was a first year, this concept was hard for me to grasp as well. The idea is that, even though the bond itself (double or triple) is stronger than a single bond as a whole, we must consider the components of a double or triple bond. A double bond consists of one sigma and one pi bond.

Let's take ethene for example. The $\ce{C=C}$ double bond has one sigma and one pi bond. The sigma exists because of two, sp2 hybridized orbitals overlapping head-to-head. This type of covalent bonding is very strong. But since it is only sp2, there is one p orbital leftover that was not hybridized. So, each carbon takes their left over p orbital, and forms a pi bond from the p orbitals overlapping side-to-side (the p orbitals are parallel). The unhybridized p orbitals are perpendicular to the sp2 orbitals.

Pi bonds, because of the way they are constructed, are much weaker than a sigma bond due to side-to-side overlap, versus head-to-head. Having said, that pi bond can be easily cleaved by an incoming group. This properties of the pi bond is what makes an unsaturated hydrocarbon so much more reactive and unstable compared to a saturated hydrocarbon.