I don't own 3D gamemaker but from what I have read about it, there isn't anyway to see the code that it generates by opening it in DBpro.
I'm sure someone will give you a definate answer on this. If you are looking to create First Person Shooters, and would like to keep coding/programming everything at a distance for now I would recommend waitng for "First Person Shooter Creator" to be released. If you are looking to create many different types od 3D games including possibly 2D apps, I would invest in DBpro. It will take you some time to have anything near a game, being it will be your first time with any programming language, but if you like it, it will stay with you for a lifetime and it is a most enjoyable hobby.
For right now, use the trial for what it was designed for. Spend thirty days with it to see if coding is for you. It's usually a "love it" or "hate it" type thing, and there isn't really any in-between.
BTW, if you are looking to create some basic 3D games, without full control of everything, The 3D Gamemaker may just be what you need. Just remember that this is a programming board, so most people here like programming languages.