I cannot find these anywhere in the Pages help menu or anywhere else. (BleedBox TrimBox and PDF/X-3) Are they in Pages? If so, where?
In the Utilities folder, find the Apple ColorSync Utility. Do yourself a favour and drag it to the Dock in order to have it on hand. It's as central to the operating system as the Displays control panel (which controls colour matching to your screen), or the Character Palette (which controls the standard character specifications you actually write), for instance.
In the ColorSync Utility, select the Filters menu and identify the Generic PDFX-3 filter. You will want to modify this filter with information from the printer you pick. An intelligent printer would publish on his web site the ICC profile for the printing conditions he supports as well as the PDF/X-3 settings for his postprocessing machinery.
An intelligent Apple management would work with intelligent printers on a country by country basis to help Apple customers. Apple development has made this possible, but Apple management is not that intelligent, unfortunately. Possibly, Apple management does not use Apple Pages to produce Apple advertising - who knows.
/hh