Area calculation using mouse clicks in game window

Hi! I saw it's possible to measure distances in game window using mouse clicks, but I can't figure out how can I measure the area, and I'm referring to a 3d scene. Ok, to measure a distance I need vector3.distance, but what should I do if I want to measure an area defined by mouse clicks, three or more points? I specify I used a plane that interacts with the mouse clicks to constrained the clicks only on that plane. Maybe you could help me, Thank you. Here is an example with what I hope, I still hope to achieve... https://s26.postimg.org/3psmrh8tl/example2_2.png