When I open 2 PDFs in DT3 and drag the thumbnail of a pages between the PDF documents, I cannot place the page I drag as the last page in the destination PDF. The last position I can reach is the last but one page.
Thanks for the report! This is a known issue and Development is aware of it. Currently, you can drop before the last page then move the last page above the second to last one. Not perfect, but certainly feasible. 
That’s exactly what I do now but as you said it is not ideal. 90% of the time I want to place the page I move to the end of the other document. But if you are already aware off the problem, I’m optimistic it will be solved soon.