Terrain Generation

Hello.

Few days ago i started thinking about making a rts game, with map generation similar to this in Civ 5 (the game won't be tile-based but i want the map to have clear planes, mountains and hills areas) so i did some research and i found few options. One is to have a script generating said terrain, but I'm afraid I can't quite pull off something like that. So I thought that I could have few prefabs like mountain, plane etc. to place and then merge them into one terrain, but i can't find a way to do it without third party software. So now here's the third option and my question: Is there a way to have some prefabs like i said earlier, and just place them on a terrain, sort of like a brush, but via code?

Thanks in advance for all the answers.

-Xentarok

P.S: If you have any questions write them down in the comments. P.S.s: English isn't my native language so I apologize for any mistakes.