As you have stated before, a related (root) problem is that teams still only play and train inside players and high OD. You also see this on the transfer list; currently there are just 6 players with 75+ TSP and 16 JR, but there are 162 players with 75+ TSP and 16 OD. I'm not surprised that in B3, there are bad 3P% games because I suspect they don't have high JR but face extremely high OD.
As long as there are no good outside shooters, no good outside shooting will happen. That needs to be fixed before we can conclude that outside shooting itself is not working correctly. BB already changed the salary impact of JR to stimulate the use/training of it, but our managers are slow to adapt (which is understandable because training takes very long). Besides that, most managers are conservative and wait with changing until they see proof it works (there are just a few early adaptors like Nachtmar and Manon). I know these are known facts but until they are solved we keep facing the problem. But how can we solve this if there are so few people willing to train JR?
One way is to do nothing and wait longer, and hope it changes gradually. This seems slow and with unsure results.
Another way is to further decrease JR salary, artificially improve it's performance in the GE or decrease the effectiveness/increase salary of OD. Could work, but with the risk that if effectiveness is improved and people also start training it more, it could get too strong.
An outside the box option would be to artificially create Free Agents with good JR, that way managers are not dependant on people training JR but can buy and expirement right away. This could be a faster and safer option than the two above, and might even help reducing the prices on the player market.