Having multiple tables does complicate this kind of report. Even though each table uses very different sets of fields, you could design a combined table and add a label field that distinguishes each type of record from another and now you can create a summary report based on this table that groups your records by whether they are Architectural items, Fixtures & Fittings, or Services.
Another options is to design a layout that uses portals for each of these tables. This approach can have some major limitations as portals limit some of your options and you have to design a layout with extremely large portals that are set to shrink to close up unsused space on your report. If you don't size your portals large enough, your report will leave out data. You also can't set individual fields within the portal to slide up either.
A third option might be to save your three reports as a PDF in a script that uses the "Append to existing PDF" option to merge the three reports into a single document.
Thanks for the reply. In the end I decided to go with your first option. I decided that the fields across all 3 tables weren't all that different, so I created a new table that combined all my records, and included an extra field that specified which type. Some fields were only used for one type, but at least I can now make a report with everything in it.