Yes, there is a way, but I have a better idea. Since this is a trial for only three months, and exporting, fixing and importing that many posts is a nightmare. I recommend, honestly, that you add a link from your current site to your install of MT3 (remember, after the free trial you pay to play) and just start adding new stuff, like a new section or expand a current old one with just a few posts. See if you like it and then decide.
When you are ready to go, then you can take on the challenge of totally converting all those posts. You can do both.
Switching is more than just importing your posts. It means checking every one, styling the new site to fit your old, writing massive redirects to accomodate the different link systems, and....trust me, I'm going through the same thing. There is a lot to consider.
So test drive and play and when you've decided, then put the energy into it.
And to get your posts out, if you are already committed, just export things from the database and then customize them to fit the MT import format.