Question

Why only 3 isomers of bromopentane are possible?

Answer 1

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 (same molecule as 5-bromopentane, but lower number is used)
2-bromopentane (same molecule as 4-bromopentane, but lower number is used)
3-bromopentane