Potential energy (Ep) is equal to the mass (m) of the subject multiplied by the height (h) from sea level that its positioned multiplied by the earth acceleration (g).
Ep = mgh
In your case m = 70 kg, g = 9.8m/s^2 , h = 3m , all the units are in SI, so simple multiplication gives Ep = 2060 J or Nm^2