I agree with the other posters here - you most probably are not going to be able to implement both the code/engine for your game and the 3D content needed in 2 weeks.
However, if the operational part of your question is "I feel that the game doesn't look very good and I want to add a 3D Graphics." then you may not need to implement a 3D engine at all in your game.
Back in the 80s and 90s, before video cards were considered standard hardware in computers, most "true" 3D games that rendered 3D on-the-fly (in software) displayed 3D as wireframes, and later flat, low-count polygons - this wasn't because people didn't know how to make "nicer" 3D graphics - the machines at the time just weren't powerful enough.
Most games were actually 2D, but used various approaches to "fake" a 3D look.
Using isometric graphics is one approach.
Another approach was create 3D models, but instead of creating a 3D engine for the game and loading/animating the model on-the-fly - pre-render graphics of them (probably using a raytracing program) into a series of images that can be animated. (i.e. pre-render the model at various angles, or at various phases of a walking animation and just string the images together.) This can be combined with the isometric approach mentioned above.
You may be able to use these ideas to make a "better looking"/"fake" 3D version of your game in the 2 weeks you mentioned - I still think it's going to be a tight schedule, but probably more do-able than learning real 3D programming and figuring out how to use a 3D modeling tool and generating 3D models for your game.