In theory yes. What happens though when we get to rounds 7-10 and there isn't 2 viable rookies per round to execute what you'd like to do?
There is already a limited number of playing rookies, which is why we all have the same ones. Some of the other better ones will potentially debut too early to be ideal as downgrades. With covid outs always an ever-present threat (particularly in colder winter), a strong bench could be crucial - meaning we don't want to be double downgrading into weak rookies to facilitate upgrades.
To my mind, if something is there at the moment that you like, then using the third trade to grab it is fine, if it strengthens your team.
A lot of those 200-300K players like Caldwell (who you were discussing. I agree, not for Berry) could be really important, as we might be stuck with them longer than we'd like. As a 4th year player with a more developed body, he should be way less likely to fall away compared to a first year player.
I think the people in the best position to use 3, are those who can downgrade a struggling premo to a legitimate replacement keeper (Cripps, Hewett, Heeney etc.), then use those funds to strengthen 2 other positions. I'm battling to find a way to do this, as I think English is my only viable option in this category.