That actually depends on how your file is formatted.
Say you have got a simple whitespace terminated ASCII file with three standard format IEEE floating point numbers in it, such as
0.73 0.1112 10.45
Then, you can read in your three coordinates like this
#include <fstream> // Header with std::[i,o]fstream classes
/* .... */
std::ifstream myfile; // Create a new input file stream
myfile.open("FILEDIR\\Filename.txt"); // Connect file stream to file
double x, y, z; // Coordinates
myfile >> x; // Read them out, one by one, whitespace automatically is ignored
myfile >> y;
myfile >> z;
Reading a different format (CSV or similar) might take a bit more time for implementation.