Is it correct to install AD DC - Exchange - ISA server on one machine .and if it's no why?

no its not  if your organization is big so you install addc in one machine and exchange server on other machine . because later you face problem any how organisation increase the load on the server machine is build to avoid this you installed exchange server on other to handle the request and manging their account that also make problem . when you facing it the messages not sending and not recieving like problem you face if addc and exchange server on same machine and hardware problem also come into picture

Yes its need good hardware requiement in your Machine for using microsoft hyper-v future in Server 2008 R2. and you can  manage those Servers in One Machine.

Actually ...I was talking about a machine without any virtualization technology....but anyway ....from my experience ...we can install AD DC & exchange on the same machine ....but we cant let ISA or Forefront join them on the same host machine....