You will need to have your PIX in routed mode.
If you use transparent mode you are assuming your internal/DMZ/outside network are all on the same layer two network (i.e. same layer two broadcast domain and I do not think this is what you are looking for). In fact, I'm pretty sure that on a PIX you can only have two interfaces per context forming a bridge, not three as it would be required in case you would like to have the three zones in the same L2 domain.
The latest and greatest ASA 8.4 supports more than two interfaces (up two four if my memory services me right) per bridge group per context but this version of the operating systems does not run on PIX hardware.
I would recommend you take a look at this document which describes in excruciating detail what it is required to do to get what you are looking for.