Sir what is the way to tell structural isomers for any compound as like C3H6BrCl

always start with degree of hydrogen defeciency (double bond equivalent – DBE). for e.g. in this case, DBE = 0, that means no double bonds are present. Now, draw the carbon skeleton (as many chain isomers as possible), identify the type of hydrogen i.e. primary, secondary or tertiary (for each chain isomers). Since, only three carbons are present in this case, so only one acyclic chain is possible and in this chain two types of hydrogen are there viz. primary and secondary. Now, start replacing hydrogen by chlorine and bromine and identify new structural isomers. 

 

In this case, total isomers comes out to be 5. (this type of questions require considerable practice, you need to perform hit and trial to find out new isomers)