Don't even try it. Finish your project in the version you started with.
CS3 does not support OMF's. ...additional, its easier to export HD on a MAC. I would like to know if there is a way to convert from a PC version of Adobe Premier to a Mac version of Adobe Premier
PC CS3 >> PC CS4 >> MAC CS4 may be the best route, but even then, not without hurdles.
Yes, you CAN. It depends somewhat on what kind of media, but in general, you can open up Premiere Pro projects on either platform. I open up Mac or PC Premiere Pro Projects on the opposite platform all of the time. However, it is always the same version (meaning CS4 and CS4 - not CS3 and CS4). Here's what I'd do to make sure it works...
Make a duplicate of the project and open it up to make sure the media is properly associated to the project on the PC side. Then take it over to the Mac. Assuming your media is .AVI, Quicktime can usually read it. If it opens, do a save as a native CS4 project.
That being said, I have heard of issues around BIG CS3 projects coming into CS4 causing some problems. If you can finish your project on PC CS3, I would.
So, yes you can do this, but it does present some challenges. Hopefully, this gives you a fairly complete answer. ???