You can use a multiple layout merge. The text frame holding your merge fields needs to be of the proper size to allow, in your case, 3-up per page. Same goes for any graphic elements.
When you go to merge, on the first tab select Multiple Records in the drop down where it says Records per Document Page, click on the tab for Multiple Record Layout and set the options for the margins, and likely the spacing between the records etc.
***Edit to add***
I do each side in separate merges and give the print establishment both PDFs.
so, I have three postcards on a 12x18 page portrait layout...I need to put a placeholder that covers all three postcards, then layout the placeholders for the persons name and address in that text placeholder? a bit confusing to me...but maybe it shouldn't be...
Nah. Your text frame itself needs to be the size you need for 3-up on your chosen page size. In simple terms that do not likely fit your circumstances would be to use a frame no more than 4" tall on a 12" x 18" portrait page. This single text frame that holds your placeholder fields will likely need to be a tad smaller.
A single text frame, positioned at the top of the page where you need it to be for the first record, and only a single set of placeholders in it.
The merge process will create the next two frames on the first page, three frames thereafter on each successive pages.
that makes a bit more sense...but what if my document has already been created three up...created with id, then pdf'd, then brought back into id three placeholders up...I do understand what you've written and that may be the method I go with...thanks for your time though...Tim
I would do it over for the merge.
ID cannot have more than one set of placeholders (well, one can have multiple instances of fields). But it may well be possible to lock down the layer with the PDF, create a new layer on top positioned carefully in a single frame, and then run the multiple record per page merge. Never tried it myself.
got it...I'll do a new document...thanks for your time..