We're aiming to set up LT - action 1, RT - action 2, LT + RT - action 3 here.
I don't think this is supported via chording at present. You'd need to handle it manually (probably via adding/removing contexts based on LT/RT press). Ideally chording would be able to handle it, but this would require a UInputTriggerChordBlocker to be added to Action1 and Action2 (currently it'll only be added to Action1, via the Action3 Chord).
Best course of action is probably to recognize Action2's key is involved in the chording (it's mapped to the chording action) and block it too, but we may need to add an opt out of such behaviour to UInputTriggerChordAction (probably defaulting to blocking the chord action key).
See this UDN for the report raising this. Seems like a fairly common use case for chording so worth supporting.
Create Action1, Action2, Action3, Action3Chord input actions.
Set up a context mapping Left Trigger to fires 1, Right Trigger to fire 2.
Add LT to fire Action3, and RT to fire Action3Chord.
Set Action3Chord to not consume input (to enable Action 2 to fire normally).
Update Action3 with a ChordAction trigger (pointed at Action3Chord).
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-149152 in the post.