Why only 3 isomers of bromopentane are possible?

There are numerous possible constitutional isomers of bromopentane.  Specifically, the bromine atom can be attached to any of the carbon atoms (although there are only three unique possibilities because the 2 end carbons are identical).  So you could have: 
1-bromopentane 
2-bromopentane   
3-bromopentane 

Then you can also have different connectivity between the carbon atoms.  So pentane can either be normal pentane (5 carbons in a row), or it can be a branched structure.  There is only one unique branched structure for pentane, which is called 2-methylbutane.  So, starting with this branched version of pentane, you could have: 
1-bromo-2-methylpentane 
2-bromo-2-methylpentane 
2-bromo-3-methylpentane 
1-bromo-3-methylpentane