1) When you configure virtual-routers, ideally you assign interfaces to it. In your scenario it would be:
set routing-instance routing-table-ISP1 interface ge-0/0/3.0
set routing-instance routing-table-ISP2 interface ge-0/0/5.0
2) With the help of rib groups, you have exported inet.0's interface routes to routing-table-ISP1.inet.0 & routing-table-ISP2.inet.0 which is sufficient for basic functionality. However if ISP1 goes down, once traffic reaches routing-table-ISP1.inet.0 routing instance, it will not be able to go to routing-table-ISP2.inet.0. For that you will need configuration like the one given below:
a) Define rib-group in each VR as below:
set routing-instance routing-table-ISP1 routing-options interface-routes rib-group inet ISP1-ISP2
set routing-instance routing-table-ISP2 routing-options interface-routes rib-group inet ISP1-ISP2
b) Import these routes to each other's VR:
set routing-options rin-groups ISP1-ISP2 import-rib [routing-table-ISP1.inet.0 routing-table-ISP2.inet.0]
3) I am assuming that following configuration exists on the box:
a) Firewall filter applied to ge-0/0/3.0 interface correctly.
b) NATing configured properly between Trust-ISP1 zones & between Trust-ISP2 zones.