If this is an out of the box Screen then do the following, as the boxes are hierarchical. Hence as you have 192.168.1.1/24 as the default ip address on e1, e1 is bound to zone trust, you must first remove the ip before you can alter zone.
NS5GT>unset int e1 ip
NS5GT>set int e1 zone v1-trust
NS5GT>set int e3 zone v1-untrust
and you should get a message stating you are running pure L2-Mode.
Have fun with the Netscreen oop Juniper NetScreen.
Kind regards
Njetscreamer