3d-trilateration: a library for three-dimensional space trilateration 
This library provides utilities to perform trilateration (also known as true range multilateration) in a three-dimensional space. In other words, it determines the location in a 3D space of a target point, given the distances from "station" points whose locations are already known. A more thorough explanation is available at the Wikipedia page for "True range multilateration".
The implementation uses the Gauss-Newton algorithm to solve a nonlinear least squares problem. In practice, the algorithm looks iteratively for estimates of the solution that are closer and closer to the actual solution. This is better suited to account for inaccuracies in the measurements of distances and locations.