There are 3 isomeric tribromobenzenes.
Their IUPAC names are:
To determine this, draw a benzene molecule and place a bromine atom at
any point and fix this as bromine 1. There are then five
possibilities for bromine 2, so repeat the next step for each
possibility. Try all the possibilities for the third bromine and then
write down the corresponding IUPAC name. Once you exhaust every
possibility (which is very fast once you pick up on the pattern) you
will have only these three isomers.
Feel free to ask for additional information or clarification.