If sigma bonds are stronger than pi bonds, why then do we say that the triple bonds which have 2 pi bonds are stronger than single bonds?
Some points to clarify:
- Sigma bonds is indeed stronger than pi bonds.
- Therefore, sigma bonds have a lower energy than pi bonds (although sigma bonds do indeed have a higher dissociation energy than pi bonds).
- Double bonds (usually) contain $1$ sigma bond and $1$ pi bond.
- Triple bonds (usually) contain $1$ sigma bond and $2$ pi bond.
From this, you can see why the triple bond has a higher (dissociation) energy than a single bond: because a triple bond contains three bonds, one of which is the sigma bond also present in the single bond.