Can you downgrade effects presets for use in Audition 3.0?

I think that the answer is generally that if you used the same effects in a later generation and try to use them, with the same settings, in an earlier one that they tend to sound different - the parameters don't always carry over. I suspect though that your largest problem will be that as far as I'm aware, the only session conversion utility around (ses2sesx) only moves things forwards, not backwards. Audition 3 can use session files in two formats - .ses and .xml - but later versions are somewhat different and created as .sesx files, which earlier versions won't open. For the ultimate confirmation of this though, you'll have to wait for SuiteSpot to notice this post - he wrote ses2sesx and is the resident guru on the subject.