Does all three Firewall contain same Os called ScreenOS ? sorry for my neglegence
Yes, the ISG, SSG and Netscreen all run ScreenOS. The newer SRX runs JUNOS.
Johhn i need to know if i make my self prepare on only one of the device device can i able to work in all of three?
There are some differences in hardware options, but nothing you couldn't pick up from looking over the documentation. Once you learn ScreenOS, you should be able to support all three.