How can I add a 3d object as a marker on Google Maps like Uber does

From what I can tell, they are not using 3D objects. They are also not animating between 400 images of a car at a different angle. They're doing a mix of rotating image assets and animating between ~50-70 images of a car at different angles. The illusion is perfect because it really does look like they used 3D car models !

Look at this GIF of a Uber car turning a corner (Dropbox link):

We can clearly see that that the shadow and the car's view angle doesn't update as often as the car's rotation.

Here I overlaid 2 images of the car at different angles, but using the same car image:

We can see that the map is rotated ~5 degrees but the car image is perfectly clear because it hasn't changed, it was simply rotated.

Uber just released a blog post documenting this.

It looks like the vehicles were modeled in 3D software and then image assets depicting different angles were exported for the app. Depending on where the vehicle is on the map and its heading then a different asset is used.

First, they are NOT 3D Objects if that's what you referring to (It's possible to create one though, but waste of time) They are simply 3D image created in Photoshop or Illustrator (Mostly) that have 3D perspective (It's also retina optimized, that's why it looks very clear).

The reason you see that the car is rotated its because the UIImageView that the image is being held into is rotated (using CABasicAnimation mostly) using calculation base off of 3D device position (Same technology use for running apps to track your location etc), which you can use Core Location to retrieve that data.

It's a proccess, but very doable. Good Luck!