..such as free 3D models. Is that allowed? If not, I'm screwed, but oh well. It's been fun anyway

From the rules:

What assets can I use?
You can use any audio or video assets you have the legal right to use. Code and design, however, must be done exclusively for the jam, during the allotted time. Except for really basic, universal scraps of code like controller support, and the like

So if the free 3d model is royalty free (which I imagine it is), then you're fine.