What formats does Quick Look for iOS support for presenting 3D information? I want to present some 3D models to my clients via email. I don't know what apps they have installed and I can't have then install any apps, most of them have a strict app policy.

What 3D formats does Preview for iOS support?
You probably should look at what is listed here: QLPreviewController
Clipped from the developer site:

A Quick Look preview controller can display previews for the following items:

iWork documents

Microsoft Office documents (Office ‘97 and newer)

Rich Text Format (RTF) documents

PDF files

Images

Text files whose uniform type identifier (UTI) conforms to the
public.text type (see Uniform Type Identifiers Reference)

Comma-separated value (csv) files



As far as I know, the only format that could embed 3D information in the format is the PDF file. But does QLPreviewController render it? I do not know.
Maybe an alternative for you would be to make a WebGL website that you link your customers to?