How does triangulation determine the epicenter?

To locate the epicenter you use the speed of waves that travel from the epicenter to the seismic sensor locations. With two sensors you are able to narrow the location to two places (when on a surface using intersecting hyperbolas). With a third sensor you have the location on a surface and below ground. This third sensor is why they call it tri - angulation but the angles are hard to find without HS trigonometry and other mathematics.