It takes to long to print from Acrobat because of flattening

I am using Adobe Acrobat Pro. I am taking a customers postcard PDF and mail merging it in Indesign. I then take the mail merged cards and set them three-up in a new Indesign file. I export this new file as a PDF and then print from Adobe Acrobat Pro. It is taking forever to print the three-up file. Acrobat has to flatten the cards before it sends the file to the Xerox printer. I am trying to find way to print from Acrobat that does not take hours to flatten the file(s). I am open to suggestions on changing the way I do this process.

Thank you in advance.