Sorry for not replying - only just seen this!
If your job is set up for example, 1,1,1, 2,2,2 3,3,3, 4,4,4, 5,5,5, 6,6,6 etc..
Set up your job in mixed media for page 1 to pick from tray 1, page 2 from tray 2 and 3 from tray 3
Then in VDP, define record length to be 3, and it will print the...