Examine the technique of the project, both how it was made and the finish. Are edges clean, are different pieces of the project attached firmly? If the project is utilitarian, like a chair, is it designed well enough to be used for its intended purpose? Are the surfaces, whether they be rough or clean, designed in line with the terms of the project?
Look at the colors used. Color may not be thought of as particularly important for three-dimensional projects, and if your project is monochrome (cut from white Bristol board, for example) then this factor should be omitted. However, all materials have an inherent color, whether wood, metal or cast materials, and these colors should be considered carefully in the design process.
See how the forms relate to each other. Is there a good contrast between different kinds of forms, such as curved and straight, or is a single form used in an interesting or innovative way? There is no formula here; you will need to rely on your aesthetic judgment to see if the shapes that make up the project are interesting and fulfill the demands of the project. Again, if there is a functional element to the assignment, the project must live up to that.
Look at the project from 360 degrees. The project, as you move around it or rotate it, should invite you to keep walking or turning the project to see what lies on the other side. A good three-dimensional project keeps you moving with forms, shapes and colors that keep the viewer interested and curious.