Welcome to the SRX.
Policies are in the format between zones From zone NAME1 to zone NAME2, with the direction being the device zone that initiates the traffic.
Zones can have multiple interfaces. So you could have all three ISP in a single zone Internet if the policies you need are not ISP specific anyway. You add the interface with sub-interface unit to the desired zone name in the configuration.
You could identify all the direction of flows you need traffic and create an initial "allow all" policy from / to that zone pair. Then you are up and running.
When there are multiple policies in a from/to zone list the FIRST policy that matches the traffic is used. That means when you are adding more specific policies to permit or deny traffic after your initial "allow all" policy, you will need to move that new policy from last (default position) to before your allow all policy using the "insert" command after creation.