It seems to me that both R3 and Rex are designed for a ship
that is meant to be stressed(can't perform actions) all the time.
In that situation, It allow said ship to gain an action when it attacks poorly.
But I can't think of any build that qualifies.
On rebel ships? yeah cant think of any.
Theoretically the Ghost would use Rex if using Hera crew.....but no defense dice to use the focus on and no room for gunner so NEVERMIND lol.