To be honest I didnt find my notes with the angle calculation of the center of the earth. Just these ones. I tried to get a formula for the epicentral distance first, to combine three of them an then find the Epicenter. I don't think it‘s right since I am not using the sphere/geographical...
My first attempt was to work with the the difference in arrival times, but that didnt account for the focus to be under the epicenter. So I tried again in combination with the angle between the stations but have not arrived at a clear solution.