Sorry, you can't. doPDF creates PDF 1.4 compatible pdfs, which should work just fine for Adobe v5.x or higher.
I don't see why you would need to have it compatible with Adobe Reader 3, given that they're now at version 9 and is a free product.
The European Commission requires that the proposal pdf files that you submit are compatible with Adobe 3.
So which x in pdf 1.x is the same as Adobe 3?
So adobe 3 would mean 1.2, which is kind of obsolete already. Where are the specifications for the european commission pdf requirements listed? Maybe that page needs revised and there's someone we could contact.
although the submission site states "For the proposal content (Part B) you must use exclusively PDF ("Portable Document Format", compatible with Adobe Acrobat Reader version 5 or higher, with embedded fonts)."
compatible with Adobe Acrobat Reader version 5 or higher, with embedded fonts
In my opinion it's ok to use doPDF, as it creates PDF 1.4 compatible with Adobe Reader 5 or higher.