Question

Why can't we have isomers of first three members of the alkane series?

Answer 1

The first three members of alkaline series are
Methane, Ethane and Propane
Methane has chemical formula CH4 , Ethane ---C2H6 and Propane C3H8
In alkanes, isomers arises when a particular compound can be represented in the form of both long straight chain and branched isomer .
The structure of methane , ethane and Propane shows that they do not have sufficient number of carbon atoms to exist in the form of branched Isomer.
Methane- 
  CH4
Ethane  --- CH3-CH3
Propane --- CH3-CH2-CH3
So first three members of alkanes can be represented by Straight chain not by Branching.

Answer 2

we cannot have isomers of first three members of the alkane series because of the flowing laws of isomers:

1. the the parent chain should have the most number of carbon atoms.

2. the branching cannot be done from the first and the last atom carbon atom of the structure.