Question

why we cannot have isomers of first three members of alkane series? pls explain clearly !

Answer 1

we cannot have structural isomers for methane because it has only one carbon atom , in ethane,it has two carbon atoms and for propane , it has three carbon atoms
so the branching structure is not possible for these members of alkane series
they can be represented only by linear structure
so the structure isomers are not possible for the same

Answer 2

first what are the three first members of alkane series?
its methane , ethane and propane.
just draw the structure of them in branch.

methane- CH4
ethane -  CH3-CH3
propane - CH3-CH2-CH3

can you put any other structure for it (in branches and rings)?
it is not possible. because there are insufficient carbon atoms necessary for branching and ring.

but if you see butane ,
CH3-CH2-CH2-CH3  ia the chain form.
this is n-butane (chain)

you can also put it in branch,
CH3 -CH-CH3
           I
        CH3
This is also known as iso butane.

hope it helps!!!!!!!!!