Question

why can't we have first 3 members of alkane series isomers??????

Answer 1

This is because no branching can be made in the first three Alkanes. Only branching induces structural Isomers in case of simple Alkanes but due to less number of carbons, it isn't possible and so, no Isomers are observed.

Answer 2

isomers are compounds with similar molecular formula but different structures.
as the first three members of alkane series cannot form different structure they do not have isomers
hope it helps!.