Q:

Which is more reactive among alkane, alkene and alkyne?

Since there are two pi bonds in alkynes - are they more reactive? Is it correct? I heard that acetylene is less reactive than ethylene.

A:

The reactivity order depends on the process(as mentioned in the comment).
1) In acid-base reactions, alkynes are the most reactive followed by alkenes and alkanes. This is due to the stability of conjugate base of alkyne by sp hybridized carbon atom. 
2)In electrophilic substitution, alkynes are less reactive than alkenes. It is because of relative stability of carbocation formed in the case of alkenes.
3)In case of hydrogenation reactions, alkynes are more reactive than alkenes.

