When you are inside the 3D interface, click "sketch" on the bottom of the tool bar.
You'll see a bar of tools appears on the bottom of the screen, you have a range of options including color, stroke, shapes, text and erase.
You can also add a note and assign the note to different team members. Make sure to hit "save" when you are done. You also have the option to download the image to your computer.
When you exit out of the 3D interface, you'll be able to see all the markups that have been made to the projects.