Pathfinding RTS (C#)

Hello programmers,

Currently I am making a RTS game I started with the basics and learned a lot about A* pathfinding in a tile based system where all passible tiles are getting checked as neigbours of the parent tile. This worked for me a couple of weeks ago when I programmed a 2D RTS game but now I would like to implement a pathfinding on 3D terrains as example if I made a terrain in 3Ds Max and imported it in Unity3D I want to generate a grid system over it like tiles so I can generate what is walkable or the opposite (if this is the best way). I don't know which technique I should use so maybe you guys can help me out with this. What I would like to create is that I have world which is created in 3Ds Max as example and then make a pathfinding to let the enemies or squats move to a certain positions on the terrain by avoiding all obstacles. And I want to keep control over it so if I make a new map or remove obstacles that it would detect it.

I hope u guys understand a little,

Thanks,

HelloIam41