\[\text{Mean Absolute Error (MAE)} = \frac{1}{n} \sum_i |y_i-\hat y_i|\] \[\text{Mean Squared Error (MSE)} = \frac{1}{n}\sum_i (y_i-\hat y_i)^2\] \[\text{Mean Absolute Percentage Error (MAPE)} = \frac{1}{n}\sum \frac{| y_i-\hat y_i |}{y_i}*100\]

This video computes the 3 metrics on a very simple example.