I can't answer your specific question for certainty, but I'm guessing that if you target the document to Acrobat 6, then Acrobat 3 won't even open the file, so your question might be moot.
I did want to clarify this point, though: Security is a relative term. No matter what you do, anyone can take a screen shot and print that. Or OCR it or whatever.
The only thing you're protecting is whether they can select the text with their cursor and press CTRL-C to easily paste it into an email in text form. But there's nothing to stop them from working with the image. If their eyeballs can see it, they can capture it.