For those who are following this thread and may have missed information posted in others, you might be interested in the exchange linked below. I don’t know why the information below hasn’t been posted in this thread. The linked post seems to confirm that DTech will not be bringing back 3 Pane View. However, a planned feature for the Navigation sidebar sounds promising (if I have read the tea leaves correctly), and suggests implementation of a ‘pseudo’ 3 Pane View. I’ve suggested that some users in this thread be invited to test the planned features to see if they mitigate the workflow issues caused by the removal of 3 Pane View, but it’s only a suggestion 
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As evidenced by the several threads (apart from this one) that have cropped up on the forum over the past six months, this is an ongoing problem for users. It’s nice to see DTech engaging with the issue, and I hope that ‘pseudo’ 3 pane view will mean 3 pane view with a different name. What I mean by that is that the ‘view’ of 3 Pane View can easily be replicated in DT3 – it’s the data management functionality that can’t.