Question

what are all the isomers and is there any general rule ?? diasterisomers, enantioisomers ??

Answer 1

If there are 3 different chiral centers then we should get 2³ = 8 number of stereoisomers. But here we'll get only four isomers. They are:

CI

If you clearly see then you'll fing there is a $\sigma$ plane of symmentry present in both 1 and 2, that is perpendicular to the plane of paper and passing through the mid carbon (Br containing). Due to this we lose two optical isomers, which if present, could be the enantiomer of these two.

In 3 and 4, for both the terminal chiral carbon configurations are same. For 3, it is R and for 4 it is S. So, Though the molecules are optically active and enantiomers to each other, the mid carbon becomes achirotopic. It's orientation becomes no longer considerable. So, again we lose two enantiomers for two orientations. What I am trying to say is depicted with the example of 3:

CICI

Both the molecules are same, we can get the other by just the rotation of one. So, the orientation of the mid carbon is not significant here.

There is no general rule on number of enantiomer and diastereomer. It totally depends on the nature of chiral centers. But you can always predict the toal number of possible stereoisomer.